Drainage by ditching is a very ancient type of excavation, and even drainage tunnels were built in prehistoric times. The purpose of this work was generally reclamation of land for agriculture.
Modern advances consist largely in the use of machinery for ditching, some improved types of pipe, and use of pumps to dewater areas that cannot be readily drained by gravity flow.
Most dry ditching is done by backhoes or wheel, ladder, or drag ditchers. In soft swamps, draglines may be best. Clamshells are used for deep and tricky work. Shallow ditches in dry ground may be dug by hoes, ditchers, front shovels, draglines, graders, and dozers of straight, angle, and other varieties.
Investigation. Before excavation of any kind is begun, the site should be inspected carefully for any conditions requiring precautionary measures. This is especially important when one is working in a developed area with buildings and existing utilities. Survey adjacent properties before excavation and, if possible, before bidding for the work. Record all defects such as cracking and settlement, so that after the excavation any claim of damages can be assessed as having been caused by the work or as a preexisting condition.
The location of existing underground utilities, namely, sewer pipes, electric lines, telephone, fuel, water, and gas, must be determined before excavation. The contractor should contact the utility owners and ask them to establish the location. When necessary, discuss removal, relocation, or service interruption with the utility owner before excavation. If the utility line is paralleling the trench and within the risky triangular wedge by a distance less than the depth plus half of the trench width, extra precautions must be taken to ensure no damage to that utility.
Vacuum Excavation. In the utility business there is a procedure known as potholing to discover where an existing utility line is located. Many government agencies are adopting regulations that require potholing so that new construction does not damage or knock out existing utility lines. The preferred method for potholing is now by vacuum excavation.
The portable equipment for this is like a large size vacuum cleaner using either high-pressure water or air, up to 1410 cfm (40 cu.m/min), to excavate a hole with the soft ground material being collected in a large tank. Depending on the machine used and soil conditions, a 12-inch (30.5-cm) square, 5-foot (1.5-m) deep pothole can be completed in 20 minutes or less. Most vacuum excavation machines are capable of digging much deeper, but utility potholes usually do not have to go more than six feet (1.8 m) deep.
Vacuum excavation was developed more than 50 years ago, but its use has been emphasized by the extensive use of horizontal directional drilling (HDD), which was discussed in Chap. 1 and detailed in Chap. 20. In connection with HDD there are two main functions for vacuum excavation: (1) removal of drilling fluid that escapes from the HDD’s pilot hole during drilling, back reaming and product installation and (2) potholing, i.e., uncovering existing buried pipe or cable so HDD crews can visualize their exact location. The vacuum excavation machines are available in many sizes, ranging from compact, self-contained systems, mounted on skids or trailers, to larger truck-mounted models.
Types. There are two types of backhoe used in ditching. We have the full-revolving hydraulic hoe, with crawler- or wheel-mounted undercarriages. And there are smaller hydraulic hoes carried on the backs of tractors (or rarely, trucks), whose arc of swing is 200° or less.
Design and operation of these various machines are discussed in Chap. 13.
The hydraulic models provide complete control of most bucket motions, with power for up and down, and pull and push. A “wrist action” bucket can be adjusted in angle during the digging pass for precise control of cutting, for tremendous breakout force on obstructions, for nonspill carrying of bucket loads to the dumping point, and for selection of place and rate of dumping.
Some crawler-mounted hydraulics have separate speed and direction control of the two tracks, a refinement that helps maneuvering in restricted space and reduces cutting up of the ground.
Width. Ditching with any machine is easiest and neatest if the trench is the same width as the bucket cut. This width is that of the bucket itself, plus any side cutters it may carry. The bucket should be wider at the front than the back, to prevent the sides from binding in the cut and to simplify dumping.
Standard bucket widths, with or without side cutters, are 18 to 42 inches (46 to 107 cm). Small machines may be as narrow as 12 inches (30 cm), and big ones 5 feet (1.5 m) wide or more.
Direction of Work. A hoe should start at, and dig away from, any obstacle it cannot cross, such as a building. If there are two such obstacles, separate starts are made at each, and an extra-work connection is made between the two trenches.
If the centerline is on a grade, working in the uphill direction makes digging more difficult, by reducing digging force and increasing the tendency of the bucket to pull the machine into the ditch.
While digging downhill is easier, the working end of the trench may fill with water if the ground is wet or the job stands unfinished during a rain. Underwater work is sloppy, inaccurate, and often unstable.
Starting. The machine is placed so that it is centered on the centerline of the ditch, with the tracks or wheels parallel to it, and the bucket extended to almost its full reach and resting on the starting point, as in Fig. 5.1(A).
Actual digging procedures with the different types of hoe are described in Chap. 13.
Briefly, the soil is taken out in layers down to the required depth. The starting point may be squared off with a vertical face from top to base. The bottom is smoothed off and checked for depth as it is made.
When the desired depth has been obtained along the space the shovel can reach, it is walked away from the ditch from 2 to 12 feet (0.6 to 3.7 m), and a section of that length excavated. Short moves are made in connection with deep ditching, cutting the bottom to an exact grade, or cutting curves; longer moves are feasible for rough, shallow work.
Curves. Curves are dug as a succession of short, straight ditches, but a skilled operator can bevel the edges to produce a smooth curve. The machine stands with its center a little outside of the centerline, and digging is done in the outer half of the bucket reach. Moves are short.
Angles. Many kinds of pipe require laying in straight lines and angles rather than curves, and trenches in which they are to be placed are dug accordingly. Angles are made by digging slightly past the angle point, then shifting the shovel to straddle the new centerline, as in (B).
Spoil Piles. Spoil from the ditch is usually piled on one side, far enough back to allow a footpath or working space between it and the ditch. If a large volume of dirt is being moved, the pile must be pushed back by the bucket as it is built, and in addition it may be necessary to allow the spoil to come to the edge. Piling on both sides is usually avoided because of backfilling work. It does serve to block off the ditch so that people are less likely to walk into it absentmindedly or in the dark, although it is not adequate barricading.
Topsoil. If topsoil is to be saved and put back on top of the other fill, it may be piled on the opposite side of the ditch from the deeper digging. If the volume of the spoil is not large, the top-soil may be placed on the same side as the fill but farther back, so that when a dozer backfills, the topsoil will be next to the blade and will reach the ditch after the fill.
Topsoil is salvaged during the digging by scraping it off first, and bringing the bucket as near the shovel as possible on the last bite. The body of the ditch is then dug, with the bucket lifted out short of its closest position. There should then be a separation between the heap of topsoil and that of fill, as in Fig. 5.2. When the shovel backs away, it can dig the pile while stripping the next section.
Sod. If sod is to be saved, it should be removed ahead of the shovel. It may be dug by hand, or cut in strips by a tractor-drawn or self-powered sod cutter. The strips left by it may be sliced in sections and piled at a safe distance by hand. The sod should be taken out at least 6 inches, and preferably 1 foot, back from the digging sidelines, to avoid damage.
Guides. Sod removal serves as an excellent and unmistakable indication of the location of the ditch; otherwise a line of pegs low enough to allow the shovel to walk over them may be used. The shovel is lined up over the ditch in the same manner as described for basements in the previous chapter, except that for ditches marked with centerlines, the dead axles should be marked to indicate bucket center, not sides.
FIGURE 5.2 Separating topsoil.
If the bottom is to follow ground contour, the bucket stick may be given a paint mark that will be even with the surface when it is straight down at the proper depth. The distance may also be marked on a stick to be used for checking.
A bottom gradient that is independent of surface levels is usually set and checked by instrument from reference points. Anything from a hand level to a laser beam may be used, depending on the conditions and the accuracy needed.
Side Digging. A hoe should be worked away from the end of the ditch that is blocked. In ditching from a house to the street, it starts at the house and finishes in the open space of the street. However, it often happens that a ditch must be dug between two buildings, or under other circumstances where both ends are blocked.
The simplest method of accomplishing the necessary turnaround is to dig the ditch from one end, then from the other, having them meet at some spot where the shovel can move off to the side. The digging of the second section should be stopped while there is still comfortable room to turn the shovel and get it out, as in Fig. 5.3(A). The shovel is then turned at right angles to the ditch and walked back into the undug space, with its center pin in the centerline of the ditch, as in (C). It then digs as closely to its tracks as possible on both sides and backs away, connecting the ditch sections by digging first on a slant and then at right angles to the trench line.
This method calls for an excavation that is usually two or more ditch-widths with a hydraulic backhoe. (See Fig. 5.4.) Sometimes such a connection can be made where extra width is needed for a manhole, pumphouse, or side ditch.
Tractor Mounting. The smaller sorts of hydraulic, mounted on the backs of wheel tractors, are usually the most economical and efficient hoes for work around buildings.
They have the advantages of light weight and rubber tires, and thus are less likely to damage lawns and paths than the heavy crawlers, either while working or while getting in and out. Accidental damage to trees and structures caused by operator mistakes is likely to be small.
FIGURE 5.3 Connecting trench sections.
Another asset is that their small buckets are usually narrower than those of larger machines. For the usual pipe or wire, they take out less soil to put back. However, in this respect they are not nearly as economical as small drag trenchers.
Their fast cycle, and the prying power of the wrist-action bucket, often enables them to do work and show production that seems greatly out of proportion to their size.
The tractors usually carry front loaders also, enabling them to backfill their ditches, carry pipe, and do miscellaneous work.
FIGURE 5.4 Hydraulics make narrower connections.
In spite of usually having two-wheel drive, these machines are seldom stuck. The weight of the hoe on the driving wheels supplies excellent traction, and if that fails, downward and outward pressure on the bucket will lift the rear wheels and push the tractor right out of a mud hole. Or the bucket can pull it out backward, if that is what is needed.
Wide Ditches. When a ditch is to be more than one bucket width, one or both edges will be slightly uneven because the bucket will move inward, toward the center pin of the shovel. Usually one side is made straight by lining the shovel to that side, and by the hacking done on the other side. If neatness is important, the ridges can be smoothed by drawing the bucket in while lightly swinging against the edge.
The full width of the ditch should be taken off in layers if it is to be dug from one position, rather than cutting one side to depth then starting on the other.
If sloping beds of shale are encountered, digging should be arranged, if possible, so that the bucket teeth will cut along the bedding planes. Shale dug in this manner at moderate depths is apt to come up in sheets, so that the ditch will be widened irregularly.
Production. The rate of ditching depends on a number of variables, including depth and width of the ditch, bucket size and efficiency, cycle time of the hoe, digging qualities of the soil, obstacles and hazards both below and aboveground, presence of rock, accuracy of grade required, and need to separate topsoil.
A ditch that is shallow, with soil piled on the edge, offers the fastest digging cycle, but the bucket is not apt to fill well. Deeper digging slows the cycle and means more soil to move, but allows better filling of the bucket. A narrow bucket does not fill as well as a wide one at any depth. A ditch that is wider than the bucket takes much more time.
Boulders and heavy roots slow the digging. Presence of buried pipes or conduits that must not be broken cause serious delays, particularly if their exact location is not known. Buildings or trees that interfere with maneuvering cut down production, as does lack of space to pile spoil.
It takes much longer to clean an irregular rock surface for blasting than to dig a clean trench to grade. Then there is the extra expense of drilling, blasting, and redigging.
The need to keep an accurate grade makes an operator work more slowly, and occasional stops are needed to check gradient or depth, unless a laser-beam setup is used. A smooth bottom finish is produced readily by a wrist-action bucket, but with some difficulty by a rigid one.
Stripping sod and topsoil separately will slow the digging from 5 to 30 percent.
When a trench needs to be braced during or immediately after the digging, production will be determined by the rate at which bracing is set, which almost always is much slower than the digging.
Here’s an example of calculating digging rate:
Assume that a ½-yard (0.38-cu.m) hoe with a 36-inch (91-cm) wide bucket, including side cutters, is digging a ditch 3 feet (0.91 m) wide and 6 feet (1.83 m) deep in common earth, with no special complications.
This ditch has a width of 1 yard (0.91 m) and a depth of 2 (1.83 m), so its cross section is 2 square yards (1.65 sq.m). There will be 2 cubic yards (1.5 cu.m) removed for each lineal yard of digging, or ⅔ yard (0.5 cu.m) per lineal foot.
This machine may have a cycle of 13 seconds, and an efficiency hour of 45 minutes. It should complete 206 cycles per hour.
The soil has a swell factor of .8 (25 percent swell). The bucket averages only four-fifths of a load in loose yards; that is, its efficiency factor is .8. Multiplying the swell factor by the efficiency factor by the ½-yard capacity of the bucket, we have
.8 × .8 × .5 = .32 bank yard (.8 × .8 × .38 = 0.24 bank cubic meter) per cycle
Multiplying 206 cycles per hour by the .32 (.24) bucket load, we have a production of 65.92, say 66, yards (49.4 say 49 cu.m) per hour. Since there is 2 cubic yards (1.5 cu.m) to each running yard of ditch, the ditching rate is 33 yards (30 m) or 99 feet [say 100 feet (30 m)] per hour.
A 30-inch (0.76 m) wide trench with a 30-inch (0.76-m) bucket would come out about the same, as what was gained in handling smaller volume would be lost in poorer bucket efficiency. However, if the ditch were 12 feet (3.66 m) deep, either bucket would probably fill well.
Clamshell. A clamshell ditches best when on the centerline. If the ditch is narrow, the tagline chains are fastened to one jaw, or for a very wide cut, to both jaws. A ditch of intermediate width is made with the chains in the one-jaw position, and the soil is taken out in layers.
Connections between ditch sections are made by attaching the tagline chains to both jaws after completion of the main ditching, and digging the connection from the side. Whole ditches may be done from the side in this manner, but it is harder to keep on the correct line. The side position is desirable in deepening an existing ditch, or in digging beside a wall.
Smooth curves may be dug either by frequent readjustments of the position of the shovel, in the same manner as with a backhoe, or by having someone on the ground twist the bucket into proper position by pushing it by hand or with a stick as it is about to touch the ground.
Dragline. The dragline is the preferred shovel for ditching in swamps, and for making ditches with sloped banks when the spoil is to be piled alongside. It works along the centerline of the ditch, as in Fig. 5.5(A), cutting the bottom and slopes in one operation. If the ditch is too wide for this, two cuts are made from the sides, as in (B) and (C).
If the fill is to be trucked away, a dragline or a backhoe may be used in this manner. Draglines may have difficulty digging hard earth that the hoe would move easily.
Front Shovel. The front shovel can dig trenches 4 to 8 feet (1.2 to 2.4 m) in depth from the top, or wide trenches from the inside. A neat ditch may be dug from the top by straddling it, if the soil is very firm or if support platforms are used. Trenching may also be done beside and parallel to the shovel’s path, but this involves quite a wide cut in proportion to depth and is difficult to trim.
Interior digging conforms in general patterns to that discussed for basements in the previous chapter. Partial swing shovels can dig narrower slots than conventional models, as they do not need space for tail swing, but they cannot load trucks behind them.
Comparisons. The backhoe is the best machine for ditches of moderate depth and width where boulders or stumps may be encountered. It will break up heavily fractured hard rock and soft or thin bedded shale, and will dig very hard soils if the bucket teeth are long and sharp. It can dig out large boulders by widening the trench as much as necessary, and dragging them up the slope toward itself. The ditch can be easily curved around boulders too large to lift or pull.
FIGURE 5.5 Wide-dragline ditch.
The clam is a slower machine but is able to dig to any depth desired, and can work close to obstructions, except those that are overhead.
Ditching Machines. Continuous-type ditching machines offer great advantages in areas where hard bedrock and boulders are rare. They dig by continuous picking and sidecasting, rather than the dig-and-dump cycle of the hoe. Refer to the trencher in Figure 14.33. This equipment can be used for wind power farms, where the utilities are put underground, and sometimes in a rocky ground condition.
These machines excavate rapidly; make a neat ditch, usually with a curved bottom which is helpful in lining up pipe; can work with less headroom, and do not need space to swing. They can dig certain classes of homogeneous soft rock which a shovel cannot, and seldom tear up banks in shale.
Medium and large machines have a number of small buckets mounted on wheels or double chains, that dump on sidecasting conveyors. Small units have a single chain, fitted with teeth that cut soil and drag it to a surface auger that sidecasts it.
Buckets may be much narrower than in hoes, and drag chains may cut slots only 4 inches (10 cm) wide. Some machines may be fitted with a carbide-toothed wheel that can chew slowly through boulders and bedrock.
Ditchers can be equipped with shoes or reels to lay tile or flexible conduits immediately behind the digging, so that shoring is not necessary.
Design, operation, and applications are discussed in Chap. 14.
Graders and Dozers. Graders can make shallow ditches with sloped sides rapidly and neatly, by the road-building processes described in Chaps. 8 and 19.
If there is no use for the excavated soil, it is usually spread and blended beside the edges.
The bulldozer can dig a wide, shallow trench from the side, as shown in Fig. 5.6(A). The volume of excavation required increases very rapidly with depth because of space needed for ramps.
When the practical limit for side excavation has been reached, the dozer can work in the ditch, pushing dirt into heaps, which it then pushes to the side, as in (B) and (C).
An angling dozer can excavate by sidecasting in the same manner as a grader, but it may be harder to keep lined up.
Stripping. Ditches frequently contain rock that is too resistant to be dug by the available equipment. Occasionally the line of work may be shifted, but usually it is necessary to blast.
Dirt and rotten rock are removed by conventional methods. Spoil should be piled far enough back to allow space for the drilling equipment, and for the shovel when it returns.
After machinery has removed the soil, the rock surface should be cleaned by hand. If the trench walls are liable to crumble and slide from drilling vibration, they should be shored up, even if depth is shallow.
Open-cut blasting is described in Chap. 9. Trench work differs chiefly in the restricted working space, and in the fact that all shots are tight. Loading must be 50 to 100 percent heavier than on wide faces.
Trenching in Basalt and Lava Rock. Trenching through this kind of rock can be done with equipment, as shown in Figure 14.27, based on Trenching Services’ experience. The rocks have compressive strengths from 12,000 to 25,000 psi (828 to 1,724 bars). This type of rock is resilient and requires a high amount of energy, but using too much energy can lead to other problems.
Trenching Services uses a Vermeer Terrain Leveler Surface Excavation piece of equipment. Managing the cutting teeth on the equipment is a daily exercise to get the best shape and replace them as needed, depending on the rock condition.
Blasting a trench leads to the problem of leaving large boulders and causing larger than necessary disruption of the landscape. Blasting methods are discussed in Chapter 9. Using the trenching machine produces square sidewalls and bottom as well as cut material suitable for backfilling the trench.
Many soils will not stand in vertical walls, so the sides of the ditches must be either sloped back or braced. A few soils will not stand on even a moderate slope, and these will require very heavy bracing.
Groundwater is the most important single factor in collapse of edges of cuts. It acts as a lubricant that enables the soil particles to move on each other readily, and exerts pressure that moves the particles toward the ditch. Sand faces may fall from this cause, or from the drying action of water draining and evaporating, so that the bond between the grains is weakened.
If water is allowed to stand in a ditch, danger of caving from flow of groundwater into it and from other dynamic forces acting in the soil is reduced. However, wave action set up by dropping of stones or chunks of dirt may cut into the walls and undermine them.
FIGURE 5.6 Bulldozer ditching.
It may happen that the upper part of a trench wall will be firm, dry material, but that a shallow layer at the bottom is waterlogged and unstable. The sides will stand for only a short time before becoming undermined by movement of the lower layer.
In general, caving of ditch sides is more apt to happen minutes, hours, or days after the digging than immediately. The usual exceptions are loose, sandy, or semiliquid muds that flow into the excavation, rather than cave or slide into it.
Stabilizing. Sloping of sides for stability is a technique chiefly used for permanent open ditches, which will be discussed later in this chapter. It is seldom used for trenches for burial of pipes because of the large amount of extra digging, the space required, or the area of pavement, lawn, or other surface disturbed.
Vertical trench walls may be stabilized by bracing, draining, freezing, or chemicals. Bracing is the most common technique, and may be required by law.
Bracing Structures. Figure 5.7(A) shows a light system of bracing or shoring used where danger of caving is slight. Planks are placed vertically in the trench at 4-foot (1.2-m) intervals, and pressed against the dirt by means of push-type turnbuckles, called sheeting jacks. Bracing timbers are inserted and the jacks removed. The planks are usually 2 or 3 inches (5 or 7.6 cm) thick, the cross braces 6 × 6 inches (15.2 × 15.2 cm) or larger. Wide ditches require heavier cross braces than narrow ones.
A heavier type of shoring is shown in (B). The sides of the ditch are lined solidly with vertical planks, called sheeting planks, kept from falling inward by horizontal beams, known as walers, which are braced to each other across the ditch by timbers. These timbers are sprung into place by forcing the walers apart by sheeting jacks.
The weight and spacing of the planks and timbers will be determined by the depth and width of the ditch, and the instability of the soil. It is possible for a usually safe soil to be dangerously soft locally, due to disturbance of underground drainage, leakage of water mains, or other causes, so an ample margin of safety should be allowed.
OSHA. The Department of Labor in the United States contains the Occupational Safety and Health Administration (OSHA), which oversees and regulates the conditions that could be hazardous for workers, including excavations. Federal law on construction standards (OSHA 2207, issued in 1990) says that employees shall be protected from excavated and other material falling or rolling into an excavation by keeping such material and equipment at least 2 feet (0.6 m) from the edge of the excavation.
FIGURE 5.7 Bracing trench sides. (Courtesy of Construction Safety Association of Ontario, Canada.)
Each employee in an excavation shall be protected from cave-ins by an adequate protective system designed to meet certain standards. If the excavation is not in solid rock and is 5 feet (1.5 m) deep, or more, there are several options. One option is to slope the sides at an angle no steeper than 1½ horizontal to 1 vertical, i.e., an angle of 34° measured from the horizontal. If the excavation is not deeper than 20 feet (6.1 m), another option is to bench the sides with the bottom lift being 4-foot (1.2-m) and above that 5-foot (1.5 m) high lifts, with the slope of the edges to the bottom being ¾ horizontal to 1 vertical.
To reduce the amount of excavation, the protective system can function by shoring the vertical sides with structural materials such as mechanical or timber systems, as shown in the Fig. 5.7. Type 2 soil is cohesive with unconfined compressive strength of 1.5 tons/square foot (0.10 bars) or greater. In the United States, where OSHA governs, that is Type A soil. In the right hand of the figure where the Canadian referral is to Type 4 soil, that would be for granular soil with uncon-fined compressive strength of 0.5 tons/sq.ft. (0.034 bars) or less, and in OSHA standards that is Type C soil. Or the shoring can be a metal hydraulic system, as shown in Fig. 5.8.
Hydraulic aluminum shores with 2-inch (51-mm) diameter cylinders, i.e. struts, can protect trenches in Type B soil 5- to 15-feet (1.5- to 4.6-m) deep and up to 12 feet (3.7 m) wide. Type B soil according to OSHA standards is cohesive with unconfined compressive strength between 1.5 and 0.5 tons/sq.ft. (0.10 and 0.034 bars). There are lightweight modular trench boxes in 8- by 10-foot (2.4- by 3.0-m) models that will protect Type B excavations 25 feet (7.6 m) deep and Type C excavations as much as 16 feet (4.9 m) deep. Each of these systems must be carefully designed to fit the earth conditions at the site.
FIGURE 5.8 Manufactured trench shield. (Courtesy of Efficiency Production, Inc.)
Soil conditions must be analyzed at least daily and the trench protection system adjusted to accommodate significant changes. The person to do this is called the “competent person” who has had training with soils and the support systems. The competent person’s firs inspection of the day should be before anyone ventures into the trench. Then, other inspections should be made during the day, especially when conditions seem to have changed.
Unconfined compressive strength of the soil can be checked by a pocket size instrument known as a penetrometer, though a skilled competent person can gauge the tons per square foot strength of the soil by the use of his or her thumb nail. If one can penetrate the soil with moderate effort half way up the thumb nail, that would indicate about 1 tsf, or in the middle of Type B soil range. If the thumb penetrates several inches into the soil, it is Type C soil, which can be molded with light finger pressure.
Design. The OSHA standards contain many design requirements that must be satisfied to meet the regulations for worker safety in the excavation. The standards give the sizes and spacing of timber cross braces (struts), wales, posts, and sheathing for various soil types and trench depths. The timbers are to be oak or the equivalent, with a bending strength not less than 850 pounds per square inch (60 kg per sq.cm). The soil types are cohesive soils with certain unconfirmed compressive strength (qu), type A having high qu as compared to type C which has low qu. These types are similar to type 2 and type 4 in the above illustrations from the Construction Safety Association of Ontario. Their type 1 is hard ground to dig, in fact, close to rock. There are other standards for aluminum hydraulic shoring that are followed by the manufacturers of that form of shoring. OSHA requires a ladder, stairway, ramp, or other means of access or egress for any excavation deeper than 4 feet (1.2 m). Whatever safe means of access or egress is used, it cannot be located farther than 25 feet (7.6 m) away from any employee.
OSHA requires the contractor to sign a Competent Person who is capable of identifying existing and predictable hazards dangerous to employees, and who has authority to take prompt measures to eliminate them. This person does not need to be an engineer, but must have training in, and be knowledgeable about, soil analysis, the use of protective systems, and the requirements of this standard.
Immediate Bracing. With any bracing the ditch is made sufficiently wider than the bucket that it can work between the struts. The ditch is dug to a depth of about 2 feet (0.6 m), full width, and the top pair of walers placed, and cross braces set with such spaces that the bucket can get down between them. Planks are set vertically touching each other outside the walers.
The shovel, preferably a clamshell, now digs a foot or two below the waler. Workers with hand tools dig the dirt out from under the vertical planks, allowing them to settle, and also remove dirt under the crossbeams which the buckets cannot reach. This dirt is piled in the middle of the ditch and is taken out by the bucket when the laborers are out of range. The shovel then digs more deeply and is followed by hand tool work. At a depth of 2 to 5 feet (0.6 to 1.5 m) below the top waler, another pair of walers is set inside the planks and braced across the ditch. Alternate excavation by shovel and hand tools, undermining and dropping of side planks outside the walers, and installation of additional beams are continued until bottom grade is reached. Ordinarily, the walers and crossbeams (struts) are either heavier or more closely spaced with increasing depth as the potential pressure increases.
If the ditch is deeper than the length of available planks, those started at the surface should be of variable length. As each one drops below the top waler, another plank is placed on top of it to follow it down. Mixed lengths make this possible without weakening the structure by having a row of these joints together.
The waler beams are also of different lengths so that both members of a pair do not end together. The joint between any two can therefore be braced against a solid beam on the other side.
Two- or 3-inch (5- or 7.6-cm) sheeting, and 6 × 6 inches (15.2 × 15.2 cm) walers and crossbeams, spaced 5 to 8 feet (1.5 to 2.4 m) apart, are strong enough for moderate depths in most soils. If more protection is needed, heavier wood may be used, additional planks can be driven outside the sheeting, or inserted inside by a complicated process of removing and replacing walers. Steel sheet piling is much stronger than wood.
Movable Bracing. When the work which is to be done in the ditch can be completed in short sections so that the ditch can be backfilled a few yards behind a backhoe, a portable bracing structure can be used. See Fig. 5.8.
It may be made up of steel or wood, and should be equipped with a tow bar or chain at the front bottom, which can be gripped by the bucket teeth. It is lowered into the first section dug, and the pipe laying or other work is done inside it while another section is dug. The shovel drags it along in the ditch whenever sufficient digging or pipe laying has been completed to justify moving it.
Such a device can result in tremendous savings. However, it cannot be used on many jobs because of the necessity of checking the work, or having it inspected. Also, if the sides should close in on it, it might be very difficult to free up for moving.
Backfilling should be done as soon as possible, as allowing the sides to cave may damage the pipe, or shift it out of line.
Flowing Banks. If the sides are so unstable that they cave or flow immediately upon being cut, the sheeting planks or sheet piling must be driven down by air hammers or pile drivers, and the dirt dug from between them afterward. Penetration and control of direction are usually best if the planks are driven only a short distance below the digging. However, mud may flow so readily that the sheeting must be down several feet below excavation level to prevent it from swelling on the bottom. All gradations between this condition and stable banks may be encountered in a short distance.
Washout Failures. Only wet ground or loose sand exerts very heavy thrusts against the shoring. Water draining down the sides of a braced trench may erode them so the sheeting moves outward, thus loosening the crossbeams or jacks and allowing them to fall, after which the sheeting can be pushed in by any movement of the banks.
Stabilization. Soft ground may be stabilized by chemical treatment, well-point pumping, drainage, or freezing, by procedures described elsewhere in this book.
Pipelines must often be built across both large and small streams.
If a wide stream or a pond has a soft bottom and is not used by ships or barges, the pipe may be laid directly on the river bottom. However, it is often necessary to protect it by burying it.
Excavation may be done by a grapple dredge; by a clamshell, dragline, or hoe on a barge made up for the purpose; by a cable excavator on the bank; or from a temporary jetty.
Digging a straight trench from a dredge or barge takes experience. Anchors and winch lines must be so arranged that the barge can both pull itself across the stream, and keep or regain position against pressure of the current. Alignment is checked by surveying instruments that may be on the barge, on the shore, or in both places.
Many streams have sufficient current to fill in a trench as fast as it is dug. At low-water periods it is usually possible to block off a substantial part of the stream channel with a fill or jetty, trench in it or just downstream from it, lay the pipe, and remove the fill. The operation is then repeated in another section. Erosion at the ends of the fill may be severe.
The jetty may be built from one bank to the stream center, and then from the other bank to connect, or a dragline may build itself an island that it moves by digging at the rear and filling the front.
The pipe may be assembled in sections as trench space becomes available; or the whole crossing may be put together at one time, dragged and floated across the water, anchored in approximate position, and sunk into the trench as sections of it are completed. Floating and sinking of the pipe are regulated by the amount of air or water in it, and/or by floats and weights.
Work of this type is subject to disastrous damage if the stream floods. If there is a little warning, equipment can be pulled back from shore-based fills, but a dragline on its own island is likely to be lost. A wire rope connection should be kept to ensure getting back the crew if the water rises too suddenly to permit rescue by boat, and to help locate and salvage the machine if it capsizes. Floating pipe may be pulled back to shore.
Because of flood danger, all possible preparations are made in advance, and the crossing pushed as rapidly as possible once it is started. The urgency depends largely on the past behavior of the stream at that time of year, but few streams are ever entirely secure against flooding.
Small Streams. It is seldom practical to ditch directly across a brook without taking precautions to keep it out of the trench. This may be done by digging a sump hole upstream, and using a pump or pumps to move the water across the ditch line and back into the stream.
A less expensive method is to confine the stream flow to pipes, and work under them. Suffi -cient pipe to accommodate the expected flow is laid in the stream across the ditch line. Sectional corrugated pipe should have seams filled with mastic to prevent drip leaks.
Dams are then built across the brook above and below the ditch area, confining the flow to the pipes. There may have to be two sets of dams, a first pair of light temporary ones toward the pipe ends to permit drying out areas where the regular dams can be built of selected soil, carefully tamped around the pipes. Bentonite might be added to the soil to improve water resistance. One dam may be made wide enough to serve as a road for machinery.
The ditch is then dug in the regular manner, under and on each side of the pipes. Water oozing into the ditch can be pumped out, diverted by well pumping, or blocked off by grouting.
When the pipe has been laid in the trench and the path or road across the stream is no longer needed, the dams are dug away and the trench backfilled, and the pipes are lifted out for reuse elsewhere.
If stream flow increases beyond pipe capacity, it will overflow the dams, fill the trench, and stop work. After the flow has subsided, the dams are repaired and the trench is pumped out. Redigging to remove washed-in soil may be necessary.
When a ditch is to be left open permanently, its sides usually must be protected by masonry or rot-resistant sheet piling, or sloped back far enough that they will not slump, cave, or wash into the bottom. If a large volume of water may flow through the trench at any time, the bottom should also be protected against erosion, unless the gradient is so flat, or the water so burdened with silt, that cutting will not occur.
Ditches with low gradients, or which carry dirty water, must be cleaned out periodically by a dragline shovel or other excavator. Masonry, riprap, and in particular vertical stone walls interfere with machine digging and are liable to be damaged. This should be borne in mind in designing any artificial protection.
Sloped Banks. The most satisfactory bank protection for a country ditch is a stable slope and a good cover of vegetation. This can be reinforced on the outside of bends and other places subject to strong current action, by placing large boulders, walls, riprap, or light piling holding wire or brush mats.
Stable slopes vary in steepness with the character of the soil. Loess may stand indefinitely in vertical cliffs, while certain types of clay may slump if the slope is 1 on 6. Generally, it can be said that slopes of 1 on 2½ or less are advisable if the soil contains much clay or silt; if there is movement of groundwater through it toward the ditch; if there is considerable drainage of surface water running down its face; or if it is in layers that dip toward the ditch.
Vegetation. If trees or bushes are to be planted or allowed to grow, the slope can be steeper than if it is to be kept in grass. Trees have greater holding power, and maintenance of grass may require the use of mowing machines, whose ability to work on side slopes may determine the grade. However, trees will interfere with access for cleaning.
If the trench is partly or wholly in barren soil, topsoil may have to be spread on the banks to encourage vegetation, although some plants generally can be found that will grow well on the subsoil if encouraged with lime or fertilizer.
Banks of permanently wet ditches may be strengthened by laying willow poles or logs up and down the bank, 2 to 4 feet (0.61 to 1.22 m) apart. They should be settled well into the soil for their full length, with the lower end in water or bottom mud, and staked or wired in place.
These poles should grow tops and a continuous root mattress.
Bottom scour may be largely prevented by keeping a gentle gradient. A grass lining will help prevent scour in a permanent ditch, as shown in Fig. 5.9. If part of the trench is too steep, check dams may be built.
This localizes the fall of the water at the erosion-resistant aprons of the dams. Good results can sometimes be obtained with plank dams, or even with heavily anchored brush mats.
Spoil Arrangements. The spoil piles from a ditch are apt to interfere with local surface drainage. If the land is flat, spoil may be piled on both sides, but frequent breaks should be made in the windrows so that water will not pond behind them. If the ditch cuts across a slope, these breaks need be made in the upper pile only. Until the ditch slope is protected by vegetation, gullies may form at these spots unless the area is protected by pipe, flumes, or stone.
The “W” ditch seen in Fig. 5.10 is a double ditch, separated by sufficient space to provide disposal area for the spoil. This eliminates any blocking of drainage on either side, and allows maintenance of field grade to the ditch edge.
If the depth of the ditch is determined by the flow capacity required, two can be shallower than one. This construction is only slightly more expensive than the single drainageway, although it usually takes more land out of production.
FIGURE 5.9 Grass-lined ditch. (Courtesy of North American Green.)
If depth is determined by a flow gradient, a W ditch will about double the amount of excavation required. If the depth is considerable, the additional spoil is likely to damage an excessive area.
Whenever possible, the spoil piles from permanent ditches should be rounded off so as to permit easy access to the ditch and to make them less prominent in the landscape. This should be done before they are overgrown by trees.
Most trenches are dug for the purpose of burying pipes or conduits. Conduits, and pipes for gas and water supply, run at more or less fixed depth below the surface. Sewers, storm drains, and other gravity-flow pipes must maintain a minimum gradient from source to outlet or booster pump, and will have a variable depth below an irregular surface,
Fixed Depth. In cold-winter areas, water pipes are laid below the frost line in the ground. Conduit and wires are laid only deep enough to protect them against accident. In either case, depth may be increased under sharp ridges so as to provide smooth vertical curves.
In fields, the most important menace is the moldboard plow, which penetrates 8 or 10 inches (20 or 25 cm). There is a chance that a subsoil plow, with a penetration of 18 inches to 2 feet (0.46 to 0.61 m), might be used; in addition, land even on gentle slopes gradually washes away, and the surface may be lowered several inches during the life of the conduit. Depths of 2 to 4 feet (0.61 to 1.22 m) are usual, and are figured from the surface of the ground regardless of slope.
Gravity Systems. Close supervision is required to ensure accurate digging for a sewer or other gravity system. A number of methods are used to keep ongrade, following the surveying methods discussed in Chap. 2.
Plotting Profiles. If there are no plans, or if they merely specify a gradient, the surface profile should be drawn to scale on a sheet or strip of cross-section paper.
In Fig. 5.11 a basement has been dug, and it is desired to lay a pipe on a slope of 1 foot (0.3 m) in 50 feet (15.2 m), to take water from a tile drain laid around the outside of the footings. An instrument is set up and the elevation of the basement floor taken. This is arbitrarily assigned a value, say 10, and is used as a benchmark for the rest of the work. Another benchmark on a tree or some surface spot not affected by building work should also be taken for future reference.
A profile is then taken along a downslope, every 25 feet (7.6 m), until points are found well below the floor level.
FIGURE 5.11 Determining the gradient.
The figures obtained are plotted on a piece of cross-section paper, which might be 10 squares to the inch. A horizontal scale of 1 inch (2.54 cm) to 25 feet (7.6 m) and a vertical scale of 1 inch (2.54 cm) to 5 feet (1.52 m) are selected. The width of each printed square will then indicate 2½ feet (0.76 m), but its height only 6 inches (15.2 cm).
The basement is sketched in, and a base or zero line drawn 20 squares below its floor. The stations (points where elevation is measured) are marked on the vertical lines, one for each inch (2.54 cm).
Each of the station elevations may now be marked on the diagram by measuring up from the baseline one square for each 6 inches (15.2 cm). These dots are connected by a line which is a picture of the surface slope, with its steepness exaggerated.
The ditch may now be drawn in. A distance of 1½ feet (0.46 m) below the basement floor is marked 3 squares down, for the tile. At the last station, 2 + 0 (200 feet), measure 8 squares [4 feet (1.2 m)] down from the tile.
A straight line drawn between these points represents a drop of 4 feet in 200, which is the 1 foot in 50 feet that is wanted.
Measurements on this sketch will now give the length of pipe needed, the distance on the ground to the outlet, the elevation of any point on the ditch bottom, and the depth of the ditch anywhere.
A larger scale in which each square would represent a smaller distance will give more accurate readings.
Finish Levels. When the digging of a ditch section has been finished, boards may be placed on edge across the ditch at 10- to 25-foot (3- to 7.6-m) intervals, and staked or otherwise firmly fastened in undisturbed soil, in position such that a tight string may be run over the ditch and adjusted by instrument readings to be parallel with the final grade of the bottom. Extra strips of wood may be nailed on, or notches cut into original boards, if they are too low or too high. Finishing of the bottom, and placement of pipe, is governed by measurements down from this string.
Laser. Lasers provide means for very exact control over line and grade in trenching, and in pipe placement.
Lasers are discussed in Chap. 2.
General Methods. Trenches dug for laying of pipes or conduits must be backfilled when the installation is complete. The dirt taken out is pushed or pulled back in. This job can be handled by most earthmoving machines, but the bulldozer is the standard tool for the purpose.
If the backfill need not be compacted from the bottom up, it may be pushed into the trench in the ways shown in Fig. 5.12. The bulldozer operates at right angles to the trench, taking as large a slice of the pile as it can handle comfortably. Dirt which drifts across the blade is left in windrows that are pushed in a separate series of passes, with poor efficiency, as in (B). Or the remaining soil is pushed parallel with the ditch into the main pile as in (C), or, when the end is reached, distributed along the ditch.
There will usually be too much soil because of the increase in volume of disturbed soil and space occupied by the pipe laid. The excess may be mounded up over the trench and partly compacted by use of a roller, or by driving the dozer or a truck along it. Full natural settlement may take as much as a year, and is liable to leave low and high spots to be graded in.
If the trench is small, it may be refilled by running an angle dozer or a grader through the pile, with the blade set to sidecast it into the ditch as in (D).
Heavy backfill may be done by a front shovel walking through the length of the pile, digging it and dumping in the ditch. A backhoe or a dragline can work from across the ditch, pulling the soil into it. A dragline’s efficiency will be greatly increased by fastening a heavy plank or other block across the mouth of the bucket so that it will not fill. Shovels often are used to move the bulk of the backfill, with a bulldozer doing the final cleanup.
When pavement along the sides of the ditch is to be preserved, the best tool is a rubber-tire loader or dozer, but light or medium crawler machines, with semigrousers or flat shoes, may be used.
Special dragline-type backfillers are often used on cross-country trenches.
Compacted Backfill. If a pavement is to be laid over the refilled trench immediately, the backfill must be carefully compacted from the bottom up. This may be done by dozing or hand-shoveling fill slowly, while workers in the trench compact it with hand or pneumatic tampers. A mechanical tamper may work from the side or straddle the trench. The top layer may be compacted by use of a trench roller with a large wheel that will fit inside the ditch, or by running any heavy machine back and forth along it.
Open-textured soils may be effectively compacted by puddling. Enough water is added to the fill to make it into mud, which, upon drying, will shrink considerably. Fine-grained soils take a long time to dry, and thus are not as readily handled in this way.
Machinery should not be run along wet trench fills as it is almost sure to get stuck in them.
Imported Backfill. Drainage trenches may be filled with better-draining, more porous material. Spoil removed in the original digging is trucked away or used in grading, and the gravel trucked in for refilling. This may be dumped in piles in and alongside the ditch, and pushed into it by a dozer or grader or hand-shoveled. If considerable work of this type is to be done, a backfilling machine may be profitably used. This carries a hopper that is moved parallel with the ditch by rubber-tire driving wheels. Trucks dump into the hopper, from which a belt carries the soil to the ditch and dumps it. The backfiller can push the truck that is dumping into it, so that the truck driver can concentrate on lifting the body at proper speed.
Both the surface and the subsurface water may be removed by a seasonal drop in groundwater level; by drainage through ditches, pipes, or siphons; by pumping; by walling off or diverting the sources of water; and very often by combinations of two or more of these methods.
The purpose of dewatering may be to promote growth of crops; to dry out swamps or other objectionable wet areas that are not designated wetlands (see Chap. 6); to stabilize slopes, foundations, and road subgrades; or to facilitate excavation for any purpose.
All of these objectives except the last are accomplished chiefly by drainage—that is, causing the unwanted water to flow away through artificial and natural channels or conduits. Pumps may be used to remove water from a sump or low point of a drainage system.
Gradients. The slope or gradient of a drain will depend on the work it has to do. In tidal marshes and other practically flat swamps, ditches with zero gradient may serve to lower the water level substantially. In general, water will flow through a flat ditch, but is easily choked by sediment, growth of weeds, and dirt falling from banks, as the water flowing through it will have little or no ability to clean it. Too steep a ditch gradient may cause erosion of the bottom, undermining of banks through stream action, and damage from depositing of mud below the discharge point.
The slope must be adjusted first to the necessities of the situation, and second to the relation between the amount of water to be carried and the nature of the soil. A bottom gradient between 1-foot (0.3-m) drop to 1,000 feet (305 m) and 2 feet (0.61 m) to 100 feet (30.5 m) is desirable under most conditions encountered.
Drainage pipes should not be flat, as costs in cleaning out sediment and debris will be very high. Low gradients can be used when the water is clean, the pipe is short and large enough to allow personnel to work in it conveniently, or there is a sharp fall at the outlet so that water will flow rapidly. Generally, the minimum gradient should be 6 inches (15 cm) to 1,000 feet (305 m), and the maximum 2 feet (0.61 m) in 100 feet (30.5 m) for land title, and 10 feet (3.1 m) in 100 (30.5 m) for tight joint pipe.
Surface Water. Surface drainage may consist of disposal of water from rain or melting snow, or lowering the water level in ponds, ditches, or swamps. It may use open channels, conduits, or both. The water is usually led to a natural stream or body of water.
Such drainage may be accomplished by deepening, enlarging, or straightening and protecting existing streambeds; by digging artificial channels; or by installing underground pipes or tunnels.
There is no definite separation between surface and subsurface drainage as they operate on different parts of the same water mass.
Trenchless Excavation. If it is not practical to ditch to install a drain or diversion pipe, boring or tunneling may be used. Trenchless excavation construction methods include all the methods of installing utility systems below grade without direct installation into an open-cut trench. These methods are differentiated from the large-diameter tunnels by several factors, such as purpose or use and diameter of excavation. Tunneling is discussed in Chap. 9.
Classification. There are three major categories of trenchless excavation: horizontal earth boring, pipe jacking, and utility tunneling. Horizontal earth boring includes methods in which the bore-hole excavation is done by mechanical means without workers inside the bore hole. See Fig. 5.13. Both pipe jacking and utility tunneling methods require workers inside the bore hole during the excavation and casing installation process. However, they are differentiated by the support structure. The pipe is the structure in one, and the other has liners installed for the structure.
FIGURE 5.13 Trenchless excavation with earth auger borer. (Courtesy of ASCE Journal of Construction Engineering Management.)
FIGURE 5.14 Trenchless excavation with earth boring machine. (Courtesy of Bor-It Mfg. Co. Inc.)
Earth Boring Methods. The horizontal auger earth boring method uses the process of simultaneously jacking the casing through the earth while removing the spoil inside the encasement, as shown in Fig. 5.14. There is the slurry rotary drilling method, which uses drill bits and tubing with fluid to remove the spoil earth, instead of augers and cutting heads. The drilling fluid is a bentonite slurry, water, or air which aids in the removal of spoil. This method of horizontal directional drilling is discussed in greater detail in Chap. 20.
To help decide what trenchless method to use for a given situation the Trenchless Technology Center at Louisiana Tech University in the United States has developed tools to assist engineers with their decision.
There is also a software program developed by Vermeer Corporation which compares the various methods of utility construction for their harmful emissions to the atmosphere. It compares: horizontal directional drilling, open-cut by excavators, or open-cut by trenchers.
Another compaction method compresses and displaces the earth surrounding the casing. This method is restricted to relatively small lines, perhaps 2 to 6 inches (5 to 15.2 cm) in diameter in compressible soil. Another is the water-jetting method, which uses the principle of soil liquefaction to make the bore hole. This is similar to methods for sinking pipe in the ground vertically. There are several other methods for horizontal earth boring, but these should be enough to give an idea of trenchless excavation.
Siphons. If a drainage line is to be used only occasionally, the expense of ditching or tunneling may be avoided by use of a siphon. This is an airtight pipe or stiff hose across the water barrier, with one end in the water to be drained, the other at a lower level (Fig. 5.15). Maximum rise above water level is about 25 feet (7.6 m). Lower lifts work better.
When the siphon pipe has been filled with water, that which is between the high point and the lower or discharge end moves down the pipe by gravity, while atmospheric pressure, acting through the pond water, pushes the shorter and lighter column of water after it. This water is in turn renewed from the pond so that movement continues until the water level drops sufficiently to outbalance the suction, or to allow air to enter the pipe; air enters through leaks or through the discharge end, or water rises around the outlet to the same level as the intake.
The rate of flow will depend chiefly on the drop between the top of the water being drained and the point where the water loses contact with the outlet end of the siphon. As a pond is drained and its level drops, the flow will become slower.
Siphons in which water moves slowly are likely to be stopped by air entering the top of the outlet. This may be prevented by putting the outlet in a box or small pool so that the opening will be under water. A slow current may also allow an air lock to form from the accumulation of air or other gases escaping from the water in the pipe, or leaks from the outside.
Very small siphons may be started by mouth suction, and a medium size by inverting it so that the ends are higher than the middle, filling it with water and holding the ends closed while placing it in position. Or a tee connection in the top may be used to pour water in, keeping the ends plugged until the pipe is full and the tee tightly plugged.
The most satisfactory way to start a large siphon is with a suction pump. A way to connect it is shown in (B). A tee is placed on the outlet end, with the side opening reduced to fit the inlet hose of a small diaphragm or centrifugal pump. Means are provided to prevent air from entering through the lower opening of the tee, by means of a check valve, a screw plug, or a piece of plywood with mud on it. With this stop in place, the pump is started and the air sucked out of the siphon so that water from the pond is drawn through it into the pump. The stop on the main pipe then opens or is removed, and the pump is shut off.
Channels. Channels may consist of natural watercourses; watercourses which have been enlarged, straightened, or paved; or artificial ditches.
A streambed may be dredged to lower its level, to increase its depth or capacity, to keep it from changing its course, or to change its course.
FIGURE 5.15 Siphon and priming pump.
Level may be lowered to drain surface water from a swamp or pond, or to provide better under-drainage for land in its vicinity.
Depth is usually increased to assist navigation, or to provide for more rapid runoff of floodwater. Widening and straightening increase capacity, often at the expense of depth.
Streams normally wander in their courses, cutting away banks in some places and building them in others. When valuable property or structures are threatened by these changes, the channel may be artificially shaped to direct the force of the water away from them. This may involve turning the water back to its original direction, or forcing it to flow in a new one.
Dredging of small streams is generally done from the banks by draglines or clamshells, and of large ones by floating dredges. The material dug may be piled on the banks, or removed by trucks or barges.
When the spoil is used to build banks to control stream direction, it must be protected by paving, masonry, rock, logs, wired brush, sod, or other material. The best emergency protection for a bank that is being washed away is drilled boulders fastened together in groups of three with steel cable.
River dredging may be planned to direct the river current so that it will do most of the excavating in the new channel.
Drainage channels are often paved to protect them from erosion or slumping, to prevent changing of course, and to increase capacity by reducing friction.
Irrigation canal pavements may be used for any of these purposes and to prevent water from leaking out of the canal into surrounding soil.
Check Dams. When the slope of a channel or gutter is so steep as to make erosion likely, it can be divided into a series of easy gradients, and separated by check dams over which the water falls steeply.
It is important that each dam have a center spillway large enough to prevent water from overtopping the edges and eroding the earth alongside. An apron is also necessary to prevent undermining.
Where elaborate structures are not practical, crude ones made out of brush and logs or loose stones may serve the purpose.
When a road or other continuous embankment crosses a stream or drainageway, it is usually carried over it on a bridge or a culvert.
These structures may be distinguished from each other on a basis of width of opening. The critical width, or span, at which a bridge becomes a culvert varies from 5 to 20 feet in different localities.
Log. Figure 5.16 shows a type of log bridge suitable for carrying a pioneer or haul road, or a driveway, across a small stream.
Sill logs are set into the bank parallel with the stream, far enough back from it to be secure against being undermined. They may be braced by bolting to stumps or driven piles, or by cables stretched to anchors behind them. This anchoring is very important, as streams often flood sufficiently to float wood bridges.
Concrete. The concrete bridges suggested here are of modest length pehaps 10 to 30 feet (3 to 9.1 m) in length. Concrete bridges consist in general of two abutments supporting a slab. The slab usually includes guardrails, and supporting ribs or stringers which may be flat or arched. The abutments are usually continued into wing walls to direct the stream through the opening and to protect the embankment against sliding or erosion.
Even small structures are quite heavy and require that the abutments rest on solid footings. The flow of the stream should not be restricted, as it might then scour out the material against the abutments and undermine them. Abutments must be strong enough to resist the horizontal thrust of the fill behind them.
Reinforcement should be used throughout the structure, and is particularly important in the slab and its ribs.
The forms for the slab must be supported on a temporary wood or steel bridge of considerable rigidity.
Bridges should be engineered for the site and conditions. Construction should not be attempted without experienced supervision.
A culvert may be made of almost any structural material. Reinforced concrete or corrugated metal pipe, and poured reinforced concrete are standard for highways and railroads. Tile and plain concrete may be used for light service. Log and timber construction are usual in pioneer and military roads.
Water passages (barrels) may be round, arched, rectangular, or in special shapes. More than one may be used.
Capacity. A culvert serves to carry the water from a drainage area or watershed of a certain size. This water includes surface runoff of rain and melted snow and ice, and whatever groundwater comes to the surface within the area.
The size of culvert opening should be determined by the amount of rain which is likely to fall in the watershed within a certain period, and the character and slope of the ground so far as they affect the percentage of water that will run off, and the speed of its flow.
Additional factors to consider are the opening required by normal stream flow before it rains; the extent to which the opening may be restricted by silting; the velocity of water in the culvert; the extent to which water not passed through it can pond against the embankment before overtopping it or damaging property behind it by flooding; and the probable damage from overtopping.
Runoff. Rate of runoff is determined by intensity of rainfall, the size and shape of the watershed, and the slope, plant cover, and permeability of the soil.
Rainfall is measured in inches, and its intensity in inches per hour, although the period of measurement may be less than an hour. For example, a rainfall of 3 inches (7.6 cm) might fall at the rate of 6 inches (15.2 cm) per hour for 30 minutes. In calculating runoff, an adjusted or equivalent rate can be used which makes allowance for variations in rate and duration.
Each watershed has a period of concentration, at the end of which the runoff is assumed to be at a maximum. This is the time required for water to flow from the farthest point in the shed to the culvert. If rainfall is continuous, and ground conditions are unchanging, the runoff at the culvert will increase from the beginning of the rain until it includes water from the whole area, after which it will continue at the same rate.
This period will be longer for long, narrow watersheds than for square or round ones of similar area.
The assumptions involved are not strictly accurate, as runoff increases as the ground becomes saturated, as water penetrating the soil emerges at lower levels, and the rate of flow is more rapid as the volume in channels becomes larger. However, there are so many variables that exact results cannot be obtained, and the average culvert is not important enough to justify an individual study of its drainage area.
The intensity of rainfall will determine the amount of water that will fall on an acre. The ground, slope, and vegetation will regulate how much of that water will flow off, and the speed of its flow. The number of acres in the watershed will determine the total amount of water delivered to the culvert. The period of concentration will determine the length of rainfall necessary to bring the area to the point of full discharge.
There are a number of formulas used in runoff calculations. These may give the volume of water to be expected, or the area in square feet of the culvert or bridge opening required. Information can also be obtained from performance of existing culverts or bridges, and observed heights of floodwater.
The value of results obtained varies with the care with which field studies are made and with a number of factors that are difficult to work out. However, for the contractor who wishes a general guide to culvert size requirements, the simplest method is best.
Figure 5.17 contains two maps showing adjusted rainfall rates in inches per hour for average requirements, and for any installation where overflow or backing up is particularly undesirable. The table supplies the number of square feet of culvert openings required to drain various areas on the basis of 1 inch (2.54 cm) of rain per hour.
To determine the size of a culvert, the drainage area is measured or estimated. Topographic or airplane maps are particularly useful for this purpose. The number of acres (square kilometers), or the next-higher figure, is selected in the left-hand column. The figure opposite this acreage, in the vertical column whose description best fits the area in question, is taken and multiplied by the rainfall rate shown for the locality by the appropriate map.
This will give the culvert area in square feet (square meters). To obtain the diameter of the proper size of round pipe, use the formula
(twice the square root of the area divided by 3.14).
The indicated size should be increased if full culvert capacity may not be available, or any local conditions (such as abnormally intense rainfall, or extremely steep and nonabsorbent slopes) indicate the need.
Even generously designed culverts may be inadequate for exceptional storms, as it is seldom economically practical to provide for them.
Sidewalls or Headwalls. Sidewalls serve to hold embankments from falling into inlet or outlet channels; to direct water into and away from the passage or barrel; to reduce turbulence and prevent undercutting of the embankment; to support the ends of the culvert, and to hold pipe sections against separating inside the fill.
The wall requirement is reduced by lengthening the pipe, as in Fig. 5.18, top. Pipe resting on the original grade, or projecting clear of the bank as in Fig. 5.19, does not usually need a wall at the outlet.
FIGURE 5.17 Culvert capacity maps and table.
Notes: (a) multiply values by 0.093 to find area in square meters; (b) multiply values by 0.00405 to find area in square kilometers; (c) multiply values given by 2.54 to find rainfall in centimeters per hour.
A sidewall is usually of reinforced concrete but may be of stone, wood, or metal. It may be of heavy construction and firmly founded to resist movement in any direction; or it may be light and superficial, so that any settlement will affect it to the same extent as the pipe.
It is most convenient to place wall footings before laying the end pipes.
Metal headwalls (Fig. 5.19), are fastened to corrugated pipe by standard couplings. They can be removed and reused if the culvert is lengthened.
Alignment. Culvert barrels should be straight under most circumstances.
It is usually desirable to use the original channel for the culvert passage and to have the culvert cross the road at right angles to the centerline.
These two objectives are often in conflict.
The original channel may be undesirable if it is crooked, crosses at a sharp angle or skew, shows rock ridges, is made up of soft mud, or has a strong flow of water. In such cases it may be more economical to dig a trench nearby, lay the pipe, and then divert the stream into it.
Right-angle alignment may be ignored if the natural channel is diagonal, and can be easily prepared for the culvert; or when excessive trenching is required to bring the stream straight across.
Referring to Fig. 5.20(C), it will be seen that a slight change in stream alignment under the road can lead to a considerable amount of digging on the side, that the new stream channel will be out of balance, and it may require mats or revetments to protect the outside banks of the curves.
FIGURE 5.18 Culvert headwalls and grades.
On the other hand, changes may involve comparatively little excavating and produce more satisfactory channels than the original.
If good alignment between stream and culvert cannot be obtained on both sides, the upstream side should be favored. When the capacity of the culvert is heavily taxed by a storm, it is advantageous to get the water into the culvert smoothly.
Gradient. It is desirable to lay the culvert on the floor of the natural channel, on the original ground surface, or in a smoothly dug ditch. This gives firmer support than fresh fill. Inequalities in channel or ground are smoothed by cutting off ridges and tamping fill into hollows.
FIGURE 5.19 Metal headwall for pipe. (Courtesy of Contech Construction Products.)
The passage should have at least a ½ percent slope, but 2 to 4 percent is preferable. It should not be over 8 or 10 percent, because of probable erosion of the bottom of the lining. The gradient of the waterway down from the discharge end should be as great as that above the inlet, for a long enough distance that it will not silt up.
If the culvert is on fill or a foundation which is expected to settle, it may be laid in a vertical curve, known as camber (see Fig. 5.18), or so as to include a vertical angle. In each case, a slight gradient is used at the inlet end and a steeper one toward the outlet. Center settlement will tend to straighten out the passage.
Disjointing. If a fill settles unevenly, or part of it moves laterally, any culvert that is in it will be put under tension. Such forces generally are not sufficient to break pipe, but they are apt to pull apart the joints.
Metal pipe is very resistant to such disruptive forces, and can be further strengthened by special joint fastenings. Short-section rigid pipe may be braced at each end by a heavy headwall, founded on underlying stable material, or may be cabled together.
Depth. Fills over a culvert, to a depth of 4 to 7 feet (1.2 to 2.1 m), protected by spreading out the weight of vehicles on the surface. Deeper fills have diminishing protective effect and impose the load of their own weight, which at great depths may be sufficient to crush the pipe.
Restricted Height. If there is not sufficient space between the channel bed and the embankment surface to install a round pipe of adequate size, two or more smaller round pipes, or low-clearance pipe with a flatter cross section or a pipe arch may be used.
Poured-concrete structures may use a flat rectangle or two or more openings separated by supporting walls.
FIGURE 5.20 Culvert alignment.
Multiple pipes should be parallel, and spaced at a distance of at least half of their outside diameters, to facilitate tamping backfill. They should not be used for streams that carry coarse debris which might choke the openings.
Handling. Corrugated pipe can be made up in any multiple of 2 feet (0.61 m). Lengths of 8 to 20 feet (2.4 to 6.1 m) are usually carried in stock, and longer or shorter ones are obtainable on special order. Shorter pieces can also be made by cutting with a torch.
Small and medium sizes are usually unloaded and placed by hand, and medium to large ones lowered with a rolling hitch or with a crane. The crane may use a hitch around each end or around the middle, with the help of a person to keep it balanced. Soft rope should be used so as not to scratch the galvanizing.
Pipe should not be dragged around on abrasive ground or scratched or banged against anything, as such abuse will damage the galvanizing and shorten the life of the metal.
It is lowered into trenches in the same manner as it is unloaded.
Foundation. The base should be shaped to fit the lower part of the pipe as closely as possible, by cutting the ground to shape, or building up with well-tamped fill. The work can be checked by placing and removing the pipe, and noting whether it was in full contact.
If the floor is rock, it should be cut from 6 inches (15.2 cm) to 3 feet (0.91 m) below pipe grade, the depth depending on the height of fill to be placed, and backfilled with earth or pea gravel. If it is mud, space should be allowed for enough pea gravel to stabilize the surface. Saplings or wire lath might be laid under the gravel to provide extra stability.
If one end of the culvert is to rest on fill and the other in a cut, the fill under it should be thoroughly tamped to avoid unequal settlement.
Placement. Each section should be placed with the longitudinal seams at the side shoulders. The cross joints should have the external part of the overlap upstream, so that if the joint is not tight, seepage will tend to move into the pipe instead of out of it.
Joints. Sections are usually fastened together by a one-piece band. This is placed under the end of the first piece, and the second is laid so that the band will overlap each by the same number of corrugations. The coupler is then drawn tight by turning down the nuts with a wrench.
This joint resists any force tending to pull it apart, being about as strong as the pipe itself. If the pipe will be subject to very severe stress, a two-piece coupler riveted to one or both sections may be used. The pipes are placed so that the collar lines up, and it is fastened with the bolts and nuts.
If the pipe is large enough to work inside [36-inch (0.91 m) or more ordinarily, but less if a small worker is available], a one-piece band may be installed in the regular way, and holes then drilled through the band and the pipes, and bolts used to strengthen the joint.
If watertightness is important, an asphalt sealer may be placed inside the band before installing, or a special coupling may be used.
Jacking. Corrugated pipe can be pushed through an ordinary fill without trenching.
Figure 5.21 shows a diagram of this process. An approach trench is dug to line the pipe up, and a heavy backstop built similar to Fig. 5.14. A trough is made across the trench at the toe of the fill, for access to the pipe. A wood frame is placed against the pipes to take the push of the jacks.
FIGURE 5.21 Jacking pipe through fill.
A person can work inside a large pipe, digging from the face to reduce the pressure needed to push it. Dirt may be loosened ahead of smaller pipes by means of augers or water jets.
Trench drills such as those described in Chap. 20 make jacking unnecessary under many conditions, and can be used for jacking when it is necessary.
Handling. The standard length of concrete pipe is 4 to 8 feet (1.22 to 2.44 m), except for land tile. The pieces may be unloaded by rolling each pipe so that it will fall from the truck endwise onto soft ground or a couple of old tires. Bell-jointed pipe should be dropped on the straight end.
Pipe up to 2 feet (0.61 m) may often be rolled and pried into place by laborers with bars and poles. However, in any size it is more convenient to use some sort of hoist.
Pipe can be handled by any lifting device with power to pick it up, and enough reach and maneuverability to place it easily. A crane is most suitable for placing it in a ditch, and a front-end loader for laying it in the open.
A pipe hook (Fig. 5.22), is very useful. It permits holding the pipe at the free end, and avoids difficulties in balancing it on a chain or sling, and in disturbing the pipe after it has been laid by withdrawing the chain. Figure 5.22(A) is alloy bar, 2 inches by 4 inches (5.1 cm × 10.2 cm), for a pipe up to 60 inches. In (B) 4-inch iron pipe will handle 24-inch concrete safely.
Eight-foot (2.44 m) lengths may have a 2-inch (5-cm) hole through the wall at the center. These can be picked up by the means of an eyebolt inserted in the hole and caught inside with a nut and washer.
Foundations. The base is shaped and tamped in much the same manner as for metal pipe.
If pipe is laid in running water, leveling the bottom may be difficult. Weighted planks or troughs of preservative-treated wood may be placed on the bottom, and carefully leveled and blocked up. Straight-sided pipe can be set on this, and only minor adjustment should be required.
If the load of fill or traffic is to be heavy, or the foundation is partly on fill and partly on rock, or is unstable, a concrete bed may be used. A stiff mixture of concrete, as wide as the trench, or in forms a foot or two wider than the outside diameter of the pipe and one-quarter of the pipe diameter in depth, is placed on a well-compacted earth base. It is roughly hollowed for the pipe, which is settled into it by rocking, or slight raising and lowering.
Bell joint pipe requires cross grooves in the bed to accommodate the oversize end.
Placement. Pipe is usually laid with the bell ends upstream. If this is the case, placement should start at the downstream end. The first pipe can be lowered level, but the others should have the plain free end slightly lower so that it can be guided into place without scraping on the bottom. This tip is arranged by inserting the hook only partway into the pipe, or by pushing down on the free end.
If a section is not held in proper position by the bed, it should be chocked securely with stones or blocks until several more sections are laid, or the culvert is completed. This makes it possible to make any necessary readjustments with less work than if fill were tamped in immediately.
It is difficult to get each joint tight without considerable practice. However, it is often possible to lay several loosely, and then push them together from an end. This may be done by a small dozer in the trench, or by a cable threaded through the culvert to a crossbeam.
Joints. Except in informal or temporary work, joints should be cemented. This is particularly important if water may pond above the outlet so that it will go through under pressure, which might force it out through open joints and cause softening and channeling of the embankment.
Small pipe is cemented by wetting the ends to be joined, and troweling a rich mortar on the upper half of the plain side and the lower half of the bell end. It is desirable to rotate the free pipe slightly, after it is in position, to spread the cement evenly. The outer surface of the upper two-thirds can be troweled off.
If the pipes are large enough for a person to work inside, the whole culvert may be placed dry. Oakum is then hand-tamped into the joint cracks, and cement or bituminous mortar applied from the inside.
Ties. If foundation conditions are such that the fill may spread and pull the pipe apart at the joints, the culvert may be held together by heavy, deep-based headwalls, or by tie lines.
Tie lines may consist of three rods, cables, or chains, hooked around the end pipes. Turn -buckles or load binders are used to tighten them. They may be internal or external, as shown in Fig. 5.23.
The inside ties will reduce culvert capa -city slightly, and may cause jamming of debris and complete stopping. However, they are accessible for inspection and tightening. Out side installations are difficult to service.
A loose cable is sometimes left inside a small-diameter culvert for use if it becomes plugged with silt. Pulling the cable back and forth will make a small hole that can be enlarged by forcing water through.
FIGURE 5.23 Cable ties on concrete pipe.
Wood Culverts. Culverts may be constructed of wood when they are for temporary use, or when time or expense prohibits obtaining more permanent materials.
Construction may be to almost any desired strength. Life expectancy will vary with the type and size of wood, preservative applied, and moisture conditions. In general, the parts that are permanently wet will have a much longer life than those that are exposed to air.
Several designs are shown in Fig. 5.24.
Casual Placement. There are many situations in which it may be unnecessary or impossible to place culverts with the care required for permanent installations. These would include light-traffic driveways and farm lanes; pioneer or access roads to be used for only a short period; and urgent construction in which it is necessary to get traffic through without delay, even at the cost of possible repair or reconstruction later.
FIGURE 5.24 Wood culverts. (Courtesy of U.S. Army Engineers.)
Good standards should be approached as closely as possible, however.
If heavy traffic will ride directly on the pipe, or very closely over it, a strong construction should be used. If silting and trash are not a problem, several small pipes will be better than one large pipe, as they are more resistant to concentrated loads and they can be provided with an adequate depth of cover more readily.
If the foundation is unstable so that a part of the culvert will sink, oversize pipe may be used to provide adequate capacity after settlement and silting. If silting can be prevented, a badly sagging pipe may act as an inverted siphon.
The pipe should be long enough not to require large headwalls, unless they can be easily built with big stones or logs available on the site. Wingwalls, where required, can be made of rocks, of saplings hammered in as piling, or of brush mats.
On wet bottoms, pea gravel or crushed stone should be used under the haunches. Ordinary earth can be used as soon as the gravel has been built up above water level, but it will not consolidate under water.
Proper placing and compacting of backfill affects both the strength of the pipe and the load it has to carry.
Load Distribution. If a round pipe lies on a hard, flat surface and is subjected to load, the entire pressure falls on the line of bottom contact. If the surface is curved, the area of contact is greatly increased and the load per square inch reduced correspondingly. As the haunches curve upward, the amount of vertical support to each square inch of surface decreases until it is zero at the widest point.
Corrugated pipe is flexible and requires horizontal support as well. A normal load tends to flatten the top and spread the sides. If the sides are held in firmly, the arch form of the load-carrying section is retained and strength kept at a maximum,
No part of the foundation or backfill touching a flexible pipe should be rigid, as the whole pipe should be able to change cross-section shape as it deflects under load, and any rigid support will cause excessive strains, particularly at the edges of the contact.
Rigid pipe receives only nominal support from the side fill.
Surface loads on soil masses are ordinarily distributed over increasing areas and reducing pressures on lower levels, as in Fig. 5.25(A). If there is a difference in bearing power of the soils within the affected cone, the more rigid soil may carry most or all of the load.
In (B), backfill has been placed loosely over an exposed or projected pipe. Settlement under traffic, or from the effects of weather, will be a fraction or percentage of its depth, so that the thinner fill over the pipe will not sink as far and will project as a surface ridge. This ridge may receive heavier loads, and be more thoroughly compacted, than the soil on each side.
If a trench is loosely filled, settlement will result in a surface trough. Vehicles bumping across this will cause heavy impact loads. If the pipe is near the surface, it may be damaged or destroyed. If deep, the loads will largely be carried by the walls.
A tightly tamped backfill should distribute loads evenly over the whole area and subject the culvert to normal loads for its depth, as in (A).
Whatever load is imposed on the immediate vicinity of the pipe will be shared by it and by the fill on each side. If the fill is tightly tamped, it will bear a larger part of the burden, relieving the top of the pipe. Vertical pressure on the side fill is partly converted into horizontal pressure against the sides of the pipe, providing support for the top load.
Tamping. Fill must be tamped under the pipe haunches. It should be free of lumps, stones, and trash, and should contain enough moisture to pack, but not enough to make it rubbery. It is placed with a hand shovel in thin layers.
The tamper should have a narrow edge to enable it to get well under the pipe and, if the trench is narrow, may require a curved handle.
Filling and tamping are done evenly on both sides of the trench, to avoid shifting the pipe to the side. It may be necessary to wedge it in place temporarily with rocks or other blocking. Such material may be left and buried if the pipe is rigid, but removed if it is flexible.
Tamping blows should not be so vigorous as to wedge the pipe out of position.
When sufficient fill has been placed that its surface is out from under the pipe, mechanical tampers can be used. Fill should be compacted to the full width of the trench, or, if the pipe is on the surface, for one pipe diameter on each side. Layers should be 4 to 6 inches (10 to 15 cm) deep.
When the pipe is nearly or wholly covered, layers of 6 to 12 inches (15 to 30 cm) may be placed, and tamping continued, until the trench is filled to grade. A compacting bucket, mounted on a backhoe, can be useful.
Side Fill. When an embankment is to be built up on each side of the pipe and above it, much of the compaction can be done by machinery first moving parallel with the pipe, then across it.
A roller working parallel must be kept far enough away so as not to exert a horizontal thrust that will move the pipe. Fill and compaction should be kept even on both sides. Since there is often only one roller available, and it may not be able to get across, the other side may be compressed with a truck or by tamping.
Soil between the rolled strips and the pipe, and between the rolled strips and over the pipe, must be thoroughly tamped.
Successive layers can be rolled closer to the pipe centerline. It is good practice to postpone crossing it until the fill is as deep as the outside diameter of the pipe, to avoid pushing it out of line.
Material is pushed to the pipe by a dozer or grader.
Loosened Backfill. A compacted embankment may be built up one diameter above the pipe, then ditched over it. This trench is filled in loosely, and layers of compacted fill are built to the top of the embankment in the regular manner.
The soft fill over the pipe causes the load to be transferred to the solid sidewalls.
If any trench containing pipe is backfilled by a dozer, care should be taken not to drop rocks on the pipe.
Farm or pioneer roads sometimes cross a shallow stream on its bottom so that vehicles must drive in the water. This type of crossing is called a ford. It may be satisfactory for light or occasional traffic, but it is subject to interruption by high water and ice, and may develop bad bottom conditions that it would be difficult to remedy.
Crusher rock, in mixed sizes from ½ to 2½ inches (1.3 to 6.4 cm), makes a good patching and paving material. If bank gravel is used, thorough raking will allow the water to take away excessive fines.
In arid regions, many watercourses are dry most of the time, but will occasionally carry such large volumes of water that adequate bridges would be very expensive. In such cases the road may run across the channel at its natural grade, with no provision being made for passing water under it. The section of road in the channel is usually heavily built, with reinforced-concrete slabs up to 2 feet (0.61 m) in thickness sometimes being used. This slab should be sloped on its upstream side, and may have a cutoff or curtain wall extending below its main mass.
Occasionally a culvert may be placed under or beside the dip, to pass small water flows, or a culvert structure may include a spillway for floodwater.
Second-class roads may cross such a streambed on graded local material that must usually be worked over after each flow of water. Roads may also run considerable distances in streambeds, as this may be the only route that is practicable without heavy blasting and grading.
Water Table. Subsurface water exists in three states or zones. The lowest of the series is hydrostatic or free groundwater. Its upper surface is known as the water table, or groundwater level. It follows the contour of the land in a general way, but tends to be farther under the surface in hills and pervious soils than in hollows and fine-grained soils. If it rises to or above the surface, it makes swamps, ponds, or springs.
The actions of this water are controlled by gravity, causing it to seek lower levels by the resistance of the soil to its movement, and by fresh supplies of water reaching it from the surface.
The water table may be static, or fluctuate only slightly, or it may shift up and down widely in response to season or rainfall.
Soil which is saturated with groundwater is usually unstable under load, will turn to mud if disturbed, and does not permit the growth of roots of most plants.
If a hole is dug below the water table, it will fill with water, unless clay seals it off.
Capillary Zone. The capillary zone lies above the water table. It may be a few inches (centi -meters) deep in coarse sand, and 8 feet (2.44 m) or more in fine-grained soils. It contains a substantial quantity of water that is held above the gravity surface by capillary attraction and other forces tending to attract and hold it in the finer soil spaces.
The amount of contained water diminishes from the bottom to the top of this zone.
Capillary movement in coarse soils is rapid, in fine ones quite slow. Raising or lowering the water table may raise or lower the capillary zone.
Medium and fine soils in the zone usually contain too much water for stability, and may be subject to frost heaving. In climates where rainfall exceeds evaporation, this zone offers the best conditions for root growth. In arid regions, the water may deposit alkali in the soil and render it unfit for cultivation.
Upper Zone. The upper or hygroscopic zone contains water which is in very thin films on the particles, or is in chemical or physical combination with them. Some of this water is hygroscopic—absorbed from the atmosphere—and is greatest in amount when humidity is high.
These small quantities of water often give the soil maximum stability, by acting as a cement or binder. Much of the water is too firmly attached to be removed by plant roots, or any method but oven baking.
This zone may also contain varying quantities of rainwater, moving downward by gravity or capillarity, or adhering to soil particles. This is available to plants and may be found in sufficient quantity to make the ground unstable.
Purpose. Subsurface drainage lowers the water table. Deep drains, or those in porous soil, will lower the capillary surface also.
Soil must be drained when its water content makes it incapable of supporting roads or other structures on it, or causes frost heaving.
Playgrounds, golf courses, and other recreational areas may require draining to dry up spots that remain wet and soft long after rains.
Farmland drainage may serve to eliminate wet spots that cannot be worked as early as the surrounding land; to speed up the drying and the warming of soil in the spring; to encourage plants to form deep root systems, with resulting increase in vigor and drought resistance; and to leach out harmful substances which may accumulate in the soil.
Methods. Groundwater level may be artificially lowered by open channels or ditches, or by buried pipe or porous material. Such pipe is generally referred to as tile, even though it might be made of other materials.
In soils that will stand on steep slopes, ditches are the most economical construction down to a depth of a few feet (meters). When wide cuts must be made to produce stable slopes, or when greater depth is required, the open ditch may involve so much excavation as to be more costly than tile.
Ditches, together with any space required for spoil, may occupy rather wide strips of land. If in farms, they cut up the fields and add to the expense of planting and cultivation. They are hazardous when near roads. In any location, they will require occasional culverts or bridges for crossings.
Ditches usually require maintenance. This may include removing silt and cave-ins, repairing erosion damage, and cleaning out vegetation. Neglect may result in general deterioration, with eventual stoppage, or expensive redigging and clearing.
FIGURE 5.26 Cone of depression.
Buried drains do not cut up the fields, or offer hazards along roadsides. However, if one becomes plugged, as a result of poor design, improper installation, or accident, it may be difficult and expensive to locate the difficulty. If the stoppage is due to general silting, it will probably be cheaper to lay a new line than to dig up and clean or repair the old one.
Choice of the type of drainage will depend on local conditions and on individual judgment.
Water slope. The porosity and bedding of the soil largely determine the depth and spacing and to some extent the size of drains required for a given project.
A pool of surface water will assume a slight but measurable slope from its inlet down to an outlet or drain. If the pool is choked with weeds and brush, water may be removed more rapidly than it can flow through the obstructions, so the level at the drain may be several inches below that at other parts of the pond as long as flow continues.
The water table may be considered to be the surface of an underground pond, obstructed in its flow by soil particles. If these particles are coarse and loosely fitted, the spaces will be large enough to allow some freedom of flow, and the slope up from an outlet of the water surface will be gradual. If the soil is fine-grained and compact, the spaces will be so small that flow will be almost stopped and the gradient down to a drain point will be very steep. This slope is called the hydraulic gradient.
Slopes will usually be steeper after rains and in wet seasons than when the surface is dry.
If the drainage is to a single pipe opening in uniform soil, the drained area will assume the shape of an inverted cone, called the cone of depression. See Fig. 5.26.
If the drainage is to a ditch or porous horizontal pipe, the shape will be a trough of roughly tri -angular cross section. The water surface and movement are shown in Fig. 5.27, and the effects of spacing in Fig. 5.28.
Drainage Layout. Figure 5.29 shows the standard patterns used for subdrainage. Where practical, the intercepting or curtain drain, as in (E), is the most economical. Figure 5.30 shows a condition where it should be used.
The natural system involves use of the natural drainageways for ditch lines, and involves minimal excavation. Difficulties may be unfavorable surface conditions for ditching; an irregular pattern which duplicates in some spots and is inadequate in others; or excessively crooked lines.
The herringbone, gridiron, and parallel systems are best suited to level or evenly sloping land. The choice will depend largely on which will most readily provide best gradients in the lines.
FIGURE 5.27 Groundwater movement.
Depth and Spacing. The table in Fig. 5.31 gives general recommendations for depth and spacing. They cannot be strictly followed in every case because of wide variations in conditions. Tile should at least be below the frost line and out of danger of crushing by machinery.
When a field is first tiled, the widest permissible spaces may be used, and additional laterals added later if they are required.
French Drains. These drains, also called rubble or blind drains, consist of a rock fill in the bottom of a trench, as in Fig. 5.32, with finer material over it, to prevent dirt from working down. The usual practice is to put the large rocks in the bottom.
The more elaborate ones in (A) and (B) will serve the same purposes as open joint pipe, but the others are not suitable for water carrying sediment, as the lack of concentrated flow will allow the spaces to fill up until the drain is blocked.
Clogging of porous material may be prevented by wrapping it in a geotextile fabric mentioned in Chap. 3. A strip of cloth is unrolled along the top of the completed ditch, then its center is pushed to the bottom. The porous material is placed to the desired height, the fabric is folded over the top, and backfilling is completed.
FIGURE 5.28 Effect of tile spacing.
FIGURE 5.29 Subsurface drainage patterns.
This cloth sifts most dirt particles out of water entering the drain.
French drains are used chiefly where supplies of suitable material are abundant.
Moles. Certain types of stiff plastic soils may be drained by opening pipelike channels in the soil. This is done by attaching a mole, which is a metal piece shaped like an elongated egg, to the heel of a subsoil plow, as in Fig. 5.33. The plow is set to penetrate to the desired depth, and the mole is dragged through the ground. It pushes the soil aside and compacts it. Under favorable conditions these tubes will stay open for 5 years or more, and may open permanent channels. In other conditions, they may close immediately or within a few months.
FIGURE 5.30 Intercepting drain.
Drainage is usually to a stream, ditch, or hole. The mole is dropped in this and pulled straight into the bank and lifted out at the upper end of the run.
A tile or metal pipe, screened with coarse mesh wire, should be placed in the outlet to protect it against erosion and plugging by entrance of small animals.
FIGURE 5.31 Depth and spacing of drainage tile. *multiply by 0.0254 to obtain depth in meters.
The simplest type of drainage by underground tiling is shown in Fig. 5.34 (A). A trench is excavated, the bottom smoothed to the desired gradient; butt joint land tile may be laid with ends touching, or with spaces up to ¼ inch; and the trench is backfilled.
Water from the affected area drains into the tile through the joints and flows inside the tile to the outlet. Under favorable conditions, such a drain may function for a very long time. However, dirt falling in through the joint and entering with the water may fill it up, or plug low spots left by subsidence of the ditch bottom.
In (B) the tile is laid on a bed of gravel. This provides a firmer foundation and a porous space into which dirt can drop through joints from the pipe. This storage space for silt may fill so that the pipe will ultimately fill also; but it may serve to trap all dirt brought in during a period of adjustment, after which little or no dirt will move.
In (C) the pipe is surrounded by gravel, which preferably should be a mixture of pieces ranging from coarse sand to ½-inch crushed stone. This serves to filter dirt out of incoming water, keeps loose dirt from reaching the pipe joints, and provides a good bedding.
Tar paper or hay can be used in connection with any of these techniques. It can be laid over the pipe, where it prevents dirt from falling in, particularly when a large opening has been caused by misalignment. The joints may be wrapped individually, or covered by a continuous strip.
It may also be placed over a gravel topping to prevent soil from working down into it. The under surface of the dirt often becomes so well stabilized that it will not cause trouble after hay has rotted out.
When the tile is laid on a curve, the wide spaces at the outside of the joints should be covered with pieces of broken tile.
Connections of branch lines may be made with sewer tile “Y”s or “T”s, or by junction pits which may be made large enough to serve as line cleanouts. A Y provides a smoother flow and larger water capacity than a T of the same size.
Cradling. If the ground is muck, or otherwise unstable, the tile should be supported by boards, as in (D). Cleated boards supporting the haunches are preferable to flat boards because of better support and more permanent alignment.
Corrugated metal pipe with perforations may be used instead of tile and cradles.
Laying Land Tile. A large part of the land or drain tile used is in farmland. The work is usually on a fairly large scale, on regular grades and with adequate space. Costs must generally be kept to a minimum.
Ditching machines are particularly adapted to a rapid sequence of operations.
Small machines, with buckets as narrow as 6 or 8 inches (15.2 or 20.3 cm), may be used for depths up to 4 feet (1.2 m). These involve minimum excavation and ensure lining up of tile. As the maximum depth is approached, it becomes more difficult to place tile accurately, and very difficult to remove stones or earth that may fall from the sides. It is usually not possible to use a tile-laying shoe. It may be inconvenient or impossible to place gravel or tar paper with the tile.
Wider buckets will eliminate these difficulties, but will increase the amount of excavation and backfill.
The tile supply is laid on the field, parallel with the ditch line, just far enough to clear the ditcher, on the side away from the intended spoil pile. Pieces are placed end to end to give the correct number, with a few extra placed at frequent intervals to make up for broken or imperfect tiles.
The tile should be placed on the ditch bottom immediately behind the ditcher to minimize the danger of “losing” the ditch through caving of the sides. The first tile should be plugged with a stone or half brick to protect the line against entrance of dirt or animals. Pieces are usually picked up and placed with an L-shaped rod of light iron. A curved bottom ditch will tend to center them, but they must be checked for alignment anyway.
Tar paper, if used, should be in a narrow continuous strip in a roll, laid over the tile.
If the ditch is wide enough to work in, the tile may be laid in the same manner or by hand. In the latter case, a picker may be used to supply tiles to the ditch worker.
Gravel is sometimes laid under or over the tile by a dump truck with a small opening in the rear gate, similar to that used for supplying automatic sand spreaders. It straddles the ditch. The gravel may pour by gravity, or may be raked or shoveled down the body floor by the person controlling the gate opening.
It is important to smooth off a bottom layer of gravel before placing tile.
Tile Shoes. If the ground is not firm enough to stand until the pipe and accessories have been laid down, a tile-laying box or shoe towed by the ditcher must be used. A number of varieties are available, many of them of only local distribution.
A tile box should be slightly narrower than the bucket side cutters. It includes the bulldozer or crumber attachment that smooths the ditch bottom behind the buckets, a pair of parallel walls that will slide between the ditch walls, and a chute on which tile may be placed to be fed by gravity or manual control into the ditch bottom.
Figure 5.35 shows a simple type of box that is operated from above. A more elaborate box in which someone can work, and which permits placing of tile, bottom and top gravel, and tar paper, is shown diagrammatically in Fig. 5.36. This is suitable for the greater depths required in irrigated fields.
Two gravel hoppers are mounted on the box, front and rear. A roll of tar paper is mounted on an axle across the inside of the box. The tile layer sits near the bottom, with his or her back to the “bulldozer,” a cleaning blade which follows the wheel. The tiles rest on a shelf in front of the layer and are replaced as they are used by someone standing on the box, who picks them up with a rod.
FIGURE 5.35 Tile-laying shoe, fed from top.
FIGURE 5.36 Tile shoe, worker inside.
As the ditcher moves forward, the blade smooths and shapes the bottom of the ditch. The front hopper spreads a strip of gravel on the bottom. The thickness of this strip is regulated by raising or lowering the hopper spout. The tile is laid on the gravel, with the ends touching. The tar paper rolls off its spool to cover the pipe, and the rear hopper deposits gravel on it.
The bottom of the rear plate may be curved to smooth over the top of the gravel, or may be set to ride several inches above it. A dozer works immediately behind the machine, backfilling. This is necessary, for if the ditch is allowed to stand open an appreciable time, one of the walls might move horizontally and slide the tile out of line.
The gravel may be piled beside the digging line, outside of the tile string, and placed in the hoppers by a small tractor front loader. A hydraulic control clamshell bucket is more efficient than the regular loader bucket, as it picks up the gravel without pushing it around.
A tile box is best adapted to a wheel ditcher, as it does not disturb its digging balance. Operation on a ladder ditcher is possible, but is more difficult as it tends to pull the bottom of the ladder backward and upward.
Backfilling. In agricultural work, it is customary to backfill with the soil dug from the ditch after placing the pipe and whatever porous material is required. However, if it is to act as an intercepting or curtain drain, imported porous material—such as gravel, sand, or corncobs—may be used to near plow depth. A top layer of native soil should be used to prevent surface water from washing it with its probable burden of silt and trash.
Whatever type of material is used next to the tile, it may be advisable to place it carefully by hand until there is no danger of the pipe sections rolling out of alignment. Compacting it immediately over the pipe (blinding) may reduce silting.
Gradients (Fall or Slope). Figure 5.37 indicates graphically the most desirable gradients for land tile of various sizes. It will be noted that up to 6-inch (15.2-cm) diameter a minimum of 1 percent is desirable, over 2 percent is optimal, and that larger sizes require flatter slopes.
Steeper slopes may give sufficient velocity to create eddies that will erode material below the joint, with possible undermining of the tile. This may result in stoppage, in blowing out to the surface, or in washing out of sections of line.
When steeper overall slopes are required by the topography, tiling may be done in a series of benches or levels, connected by inclines of sewer tile with cemented joints.
Flatter slopes increase the danger of silting.
Tile Size. Figure 5.38 shows the number of acres (square meters) that will be drained by tile of various sizes, at slopes of up to 1 percent. Some of the sizes listed may not be generally available, and these figures are for average conditions.
It is good practice to use tile of larger than minimum diameter for a line so that effectiveness will not be lost as readily by silting or misalignment.
Surface Inlets. A tile drainage system may include one or more places where surface water can drain into it. The flow of water should be calculated and the tile increased in size to accommodate it.
Such inlets require careful design. They must be arranged so that dirt cannot be washed in, so that animals cannot enter, and so that hydraulic pressure cannot be exerted on the tile underneath the opening.
For example, if a 6-inch (15.2-cm) inlet to a 6-inch (15.2-cm) line is used, water ponding over the inlet due to excessive rainfall may put hydraulic pressure on the underground pipe, causing too fast a flow and probable erosion.
If the inlet is choked down to 3 or 4 inches (7.6 or 10 cm), or provisions are made for surface overflow, this difficulty should not occur.
Outlets. The tile line or system may terminate in a drainage ditch; a large drain system; a river, lake, or sea; or a sump. If a sump, an automatic electric pump should be used to keep the water level below the bottom of the tile to ensure free drainage.
FIGURE 5.39 Projecting drainage outlet.
A projecting metal pipe may be used, as in Fig. 5.39, or a masonry spillway, as in Fig. 5.40, to avoid bank erosion and undermining. Since outfall lines often lie under surface channels, it may be necessary to make provision for surface flow also, as shown.
The outlet should be protected against entrance of small animals by coarse-mesh wire, a grating, or an automatic gate.
A drainage system that ends in salt water between high and low tides, or in a waterway subject to flooding, should be fitted with a check valve or gate that will let the drain water out, but not allow the seawater or floodwater in. To make this effective, the last few feet of tile should be glazed and a concrete headwall used.
Other Pipe. Lightweight, perforated, corrugated-metal pipe may be used for tiling. It has four parallel rows of 
or 3/8-inch (7.9- or 9.5-mm) holes, and is laid so that they are in the haunches. Joints have collars to preserve alignment.
This pipe is too expensive for ordinary agricultural work, but is widely used for road subdrains because of its resistance to crushing and its dependable alignment.
Perforated asphaltic pipe is even lighter and is cheaper, but it is less strong. Both types are made in long pieces and are easy to lay, except where caving soil requires use of a tile-laying box.
FIGURE 5.40 Outlet for tile, and weir for surface drainage.
Plugging. Tiles may become partially plugged with mud entering with groundwater, particularly when no protecting gravel and paper are used. Flooded streams may also cause plugging, by either backing up the tile or causing the water to stagnate in them.
It is sometimes possible to flush tiles clean by utilizing a surface inlet, or opening the upper end and putting a large volume of clean water through. The flow should be started slowly, as a rush of water might move enough mud to form a solid block, which would necessitate abandonment of the line.
It usually is cheaper to lay new tile than to dig up and clean an old one, unless the stoppage is in a small area and can be located accurately.
Vertical drains of sand or porous soil can be used to move water up or down. They are often made by drilling or blasting holes in an impervious layer lying over a pervious one. Figure 5.41(A) shows a section of ground in which opening up the hardpan would allow water to drain into underlying gravel, and (B) shows a pond which is able to exist above the water table in a pervious sand, because silt and organic matter deposited from the water have sealed all spaces in a thin layer on its bottom and sides. Breaking this layer by any means will cause the pond to drain, unless sufficient silt is stirred up to seal it again.
Vertical drains, through which water rises because of being displaced by the weight of fill, are used to stabilize deep layers of saturated peat. Such soils may contain 50 percent or more water, and behave as viscous fluids under pressure. Fills placed on them sink, owing in part to water being squeezed out of the mud, and in part to the mud being displaced to each side. This settling of the fill may continue for a great many years, and become so uneven as to make pavements or other structures on the fill become unusable.
The squeezing out of water may be accelerated, and the sideward movement of mud practically eliminated, by making vertical holes in the mud, filling them with clean sand, and connecting their tops with a drainage system, as indicated by diagrams in Fig. 5.42. The fill is then placed, and its weight causes the water to enter the sand columns and rise into the drains. If the columns are properly spaced, sufficient water will be removed to stabilize the mud sufficiently to carry the intended load.
FIGURE 5.41 Perched water tables.
FIGURE 5.42 Vertical sand drain installation.
Part of the fill may be placed on the swamp, then the holes made by sinking a hollow-walled pipe by jetting with water and compressed air, or by driving a hollow pile with a detachable head. In either case, when the tube has reached the bottom of the soft layer, it is filled with sand and pulled, leaving the sand in the hole. Hole diameters of 16 to 24 inches (0.41 to 0.61 m) are commonly used, with spacing varying between 8 and 22 feet (2.44 and 6.71 m).
If the tube is pulled by conventional methods, the sand may stick to it and be raised sufficiently to allow mud to enter crevices in it and interrupt the drainage. This may be avoided by attaching an airtight head to the tube after the sand has been placed, and pumping compressed air between the head and the sand. This raises the tube but exerts an equal downward force against the sand and holds it in place and together.
The tops of the columns may be drained by spreading a blanket of clean gravel or sand, a foot (0.305 m) or more in depth, over the whole area; or by connecting the columns with tile or rubble drains. If the fill is all gravel, a drainage layer may not be needed.
Settling may be still further speeded by placing more fill than will be required for final grade. The extra weight will squeeze the water out of the mud more quickly. When settlement is judged to be complete, the extra fill is removed.
It is desirable to remove surface water and groundwater from areas to be excavated, but the cost may exceed the advantages gained.
Water may be removed naturally by seasonal change, or artificially by diversion, draining, siphoning, or pumping.
Seasonal Lowering. The seasonal decline in the groundwater level may be quite considerable in areas having dry summers. Some places that are so wet in winter and spring as to be very expensive to work, may become dry to depths of 5 to 30 feet (1.5 to 9.1 m). Permanent swamps may develop crusts sufficient to allow movement of machinery.
Such changes are not uniform, as a wet season may keep water levels abnormally high while an exceptional drought will cause extreme or unseasonable lowering.
When it can be arranged, it is obviously good practice to undertake wet excavation in a dry period, as any reduction in either mud or unwanted water will reduce costs. The economy is greatest in work in marshy areas and in shallow excavations, but may be noticeable even in deep work.
Dewatering of excavations commonly requires the use of pumps.
If the water is small in volume or contains a heavy load of mud or other solids, a diaphragm pump is preferred. A centrifugal pump is needed for larger quantities. A centrifugal pump should be placed as near the water level as possible. More energy is used pumping water over a high bank than over a low one, but the total lift may be largely determined by the job. However, these pumps will push the water more efficiently than they will pull it, and much better output will be obtained by keeping the suction line short.
More details about the pumps can be found in Chap. 21.
Inlet Protection. The low end of the inlet pipe should be fitted with a screen that will prevent entry of any object large enough to plug the pipe or damage the pump. Water containing leaves or other fibrous matter will clog such a screen readily, and may make necessary the placing of an outer screen of ¼- or ½-inch (0.64- or 1.27-cm) mesh, or some similar wire. This outer screen is best located far enough from the inlet that water going through it will not have force enough to hold rubbish against it to block it. See Fig. 5.43.
Another inlet problem is that of the pipe and screen sinking into a muddy bottom and becoming blocked, or being buried by soil washed over them. Placing the inlet in a wooden box will prevent sinking and make it easier for the pump to suck up washings. See Fig. 5.44.
In long inlet lines, or large ones, it is good to have a foot valve in the bottom to hold water in the pipe while the pump is not running, unless the pump is tight and the inlet is underwater at all times. This will save time pumping out air each time the pump is started. Unfortunately, foot valves are subject to jamming and sticking, and may need frequent attention to keep them working.
FIGURE 5.43 Trash protection for inlet.
A whirlpool may form over the suction end of the inlet pipe, which will allow air to enter the pipe through several feet of water. This is most apt to happen if the pipe is lying in a nearly horizontal position. It can usually be prevented by arranging the end of the pipe to hang vertically or attaching a shield over the inlet, or by throwing a square or round piece of flat wood in the water, which will tend to center in the whirlpool and block the air passage.
A pump will work best if a foot or two of water is kept over the inlet. In most excavations, the bottoms should be kept as dry as possible. It therefore is advisable to dig a sump pit for the pump hose, and to cut through any ridges that prevent water from flowing into it from the pit.
FIGURE 5.44 Mud protection for inlet.
Dirty Water. If the water flowing into the excavation is dirty, it indicates that soil is being brought in from outside the excavation. Continued pumping may cause caving of banks due to undermining; or even may cause sinking of adjoining buildings or roads. It is wise to keep such pumping to the minimum required for the work and to finish the job as rapidly as possible, even at extra expense. It may be necessary to dry the area by well points, or to block the water off by grout, chemicals, or freezing.
Contractors’ liability and property damage insurance ordinarily does not cover damage to structures by undermining, even in the “comprehensive” policies. A special endorsement is necessary, and inspection of the job is usually required.
Well Points. A well-point pump is a centrifugal pump with rather close-fitting parts, and often with an auxiliary air-vacuum pump, and which can work efficiently in spite of a fairly high proportion of air in the intake lines.
A well point is a section of finely perforated pipe that is sunk into the ground by jetting, driving, or drilling. It is attached to ordinary iron pipe, which rises to the surface and is connected by other lines to a pump that usually takes care of a number of points.
When the pump is running, the groundwater in contact with the well point is drawn through the holes of slits into the pipe and pumped away. The holes are so small that only very fine particles of earth will pass through them. The continued suction gradually removes all such particles from the area immediately around the pipe, leaving the coarser ones. This makes a porous screen with an outside area several times larger than that of the point, and improves its gathering efficiency.
Each well point will remove groundwater from a cone of depression around it, the slope of which depends largely on the porosity of the soil.
If well points are placed in a line so that their cones overlap, a continuous band of soil can be dewatered, as in Fig. 5.45.
A ditch could be dug in this band without encountering groundwater, regardless of otherwise saturated conditions throughout the area.
The well points may also be set in a square pattern to dry up the ground for a basement or similar excavation. It sometimes is possible to dewater such an area by using points as a curtain drain where the source and depth of the water are known.
In order to eliminate mud difficulties, the points should be placed deeply enough that the excavation will not reach capillary water standing above the artificially lowered water table.
In a deep excavation, it will be necessary to reset the well points and pumps on successively lower benches as the digging progresses, because of the inefficiency of high suction lifts. This may be done by starting the excavation oversize, so as to leave a shelf at the bottom of each cut for placement of the pumps and lines, as in Fig. 5.46.
Well points are most efficient in porous soils and will ordinarily not give good results in clay soils. In peat, the points are jetted down, and sand is dumped in the hole around them to increase contact area.
Proper use of well points involves considerable work in placing, connecting, and moving points, and pumping usually is on a 24-hour day basis for the duration of the job. In addition, considerable experience is desirable in order to avoid wasted time and possible failure to keep the job dry. It generally is advisable to subcontract this work to specialists.
Deep Hole Pumping. An excavation area may be predrained by sinking a number of shafts, lined with timber or pipe, and pumping from the bottom. The pumps used are usually small, with electric motors. The shafts are more widely spaced than well points and can be used to much greater depths. Drilling and lining are expensive.
Deep well pumps, or the piston or the jet type used for water supply, may be used if equipped with good sand filters.
Sump Pumping. Shallow layers of soil may be dried by digging a deep hole in the area and keeping it pumped out. Effectiveness and promptness of drying may be improved by a system of ditches draining into the sump. These may run along the outside edges of the site and into the interior in any convenient pattern.
This is an excellent and inexpensive way to dewater a swamp before digging a pond, unless the flow of water into the area requires an excessive amount of pumping during the drying process.
Jetting. Jetting with high-pressure water, or less commonly, compressed air, is used in making deep narrow holes for setting piles, installing vertical drains, obtaining soil samples, and various other purposes.
FIGURE 5.46 Deep well-point pumping.
Pressures required range from a few pounds (kilograms) over atmospheric for penetrating loose fine deposits to several hundred for tough clays.
A single pipe with a nozzle or reduction in size at the tip may be used in probing for rock or other obstructions. The tip reduction increases the velocity of the water and makes plugging less likely if it is forced into soil that the water will not cut.
Single pipe holes are irregular in shape, as the exhaust water and spoil rise around the pipe and will erode channels along the path of least resistance.
A better system is to use several water jets around the rim of a pipe so that washings can rise through the pipe to the surface. Water may be supplied through separate pipes, or by welding one pipe inside another, leaving a space between them for passage of water from an upper inlet connection to the bottom jets.
There should be at least three jets, preferably four or more. They must be evenly spaced around the circumference to prevent the pipe from drifting sideward toward the most effective erosion.
The pipe should be handled by a crane or some other type of hoist.
The lower end is sometimes fitted with teeth, and is lifted and dropped to loosen hard materials. The nozzles must be well protected against contact with hard dirt, if this method is used.
Excavating contractors are often consulted about the feasibility of having a basement under a house. The problem may be one of the cost of dealing with rock on the site, or a fear of water conditions which would make the basement wet and unusable.
If proper procedures are followed, a basement can be kept dry in any location where water does not spill in the windows or over the top of its wall. The cost ranges from sometimes nominal expense of installing subdrains to the great expense for complete waterproofing of floor and walls.
Soils and Locations. The tight soils such as clay or the various varieties of hardpan, tend to become saturated in wet seasons, even near hilltops. The quantity of water they may carry, which is the basis for deciding on drain size, may be very difficult to determine in a dry season. In general, if the soil contains long streaks or lenses of sand or very fine pebbles, it may be assumed that there is considerable flow through it. If there are spots near the building site which ooze water in the spring, or in which water-loving plants grow, a serious drainage problem is indicated.
Difficulties are sometimes avoided by shifting the building site to a spot with better drainage, or doing only shallow excavation and obtaining depth by filling around the walls. Drains should still be used, as groundwater may rise into the fill.
Subdrainage. Drainage around the footings is a precaution that should always be taken if there is any lower point to which water can be led. A porous soil such as sand or gravel can seldom hold enough water to wet a basement, but it may be part of a waterlogged lowland or a gradual slope up from one, or have water held in it by layers or lenses of clay.
The standard basement subdrainage consists of a line of land tile laid completely around the outside of the footing, and preferably 1 foot to 18 inches (0.30 to 0.46 m) below basement floor level. It should be laid in a fine-crushed stone, protected with tar paper or hay, and backfilled promptly. Such tile has a downward pitch of ½ to 1 percent from a point opposite the outlet.
The outlet may be land tile, but because of the danger of entrance of plant roots, glazed sewer tile with cemented joints is better. It should slope down away from the building at 1 to 5 percent grade to a disposal point. This may be a stormwater drain under the street, a stream, or lower ground.
A stormwater drain complication is that water entering the system at higher levels may back up through the tile and saturate the ground around its walls temporarily.
When there is no stormwater drain, or connection to it is considered unwise, a discharge point on the same property should be sought. It is often easy to get permission to lay pipe through a neighbor’s yard, but impossible to get a formal easement to keep it there.
A pipe having an open discharge should always be kept covered with coarse screening to prevent animals from entering it. If the pipe is large, a flap gate can be used, but these are not satisfactory in small sizes.
In many situations an open flow of water from a pipe is objectionable. In such cases the drain may lead to a dry well, or into tile laid out in the same manner as a septic field. An overflow exit may be provided, or the water may be left to work that out for itself.
Standard practice calls for 4-inch (10-cm) tile around the foundation. This is often too small, and its inadequacy is the cause of endless trouble. Even a small building can block a considerable area of horizontal movement of groundwater, which will try to enter it unless it is drained off. After a heavy rain, a previously unnoticed seepage vein or group of small channels may carry more water than a small tile can hope to accommodate, and the foundation wall may cut into a number of them. Six inches (15.2 cm) is a safer minimum size.
The outlet drain can be the same size if the slope is steeper, or the next larger if it is the same slope.
If the building is on a slope most, although not necessarily all, of the water will be against the upper wall or walls, and may require 8- to 12-inch (20.3- to 30.5-cm) pipe. If there is a long slope above the building, surface water may constitute a serious problem that is best solved by leading it through gratings and vertical pipes to the footing tile, which may then be as large as 20 inches (50.8 cm). The size needed can be figured in the same way shown earlier for culverts, plus a liberal allowance for underground flow.
The floor should be laid on 4 to 8 inches (10 to 20 cm) of crushed stone or gravel. This should be connected to the outside subdrain by a tile through or under the footing. If less gravel is used, one or two lines of tile might be laid under the floor. The gravel will serve to catch any vertical seepage of water, and will insulate and strengthen the floor also. The tile serves only for drainage.
Gutter leaders can be tied directly into the footing tile, emptied into nearby dry wells which will ultimately drain into it, or provided with dry wells or outlets at a distance.
If they dump directly into the tile, it must be large enough to carry easily all the water that will enter it from the ground and from the gutters. If the gutter water tries to enter a tile which is already full, it will accumulate in the leader and may build up to a head of 15 or more feet (4.6 or more meters). The resulting pressure inside the tile will force water out of the joints and cause erosion and misalignment that may result in entire failure.
On the other hand, if tile size is ample, the swift current of gutter water will tend to carry away dirt which may work into the tile from the ground.
If dry wells are used, it is best to place them well away from the building.
Where porous fill is available, it should be used for backfill against the foundation to prevent water pockets from forming against the wall. Surface water can be kept out of it by sloping up toward the foundation, and by placing a capping of topsoil.
Porous breather pipes or a hollow-tile outer wall may be carried from the tile to the surface, against the foundation. Air chilled by contact with the ground tends to flow down the drain to the outlet; and when it can be replaced by warm air pulled down from the surface, the resulting circulation warms the soil and the wall, and reduces the problem of condensation inside the building.
A building which has been built without subdrains or with inadequate ones can often be protected by installing a deep curtain drain along the uphill slope. This may be built to cut off underground water only, or to take care of surface water as well.
If a basement is to be built in a hole blasted out of sold rock, it is good practice to provide a sump hole, about 3 feet (0.91 m) deep and square, in a corner.
If water difficulties develop, an automatic pump can be quickly installed to remove it from the basement itself, or from the floor base where it might otherwise build up enough pressure to cause heaving and cracking.
The sump can be protected with a manhole cover when not in use. A 36-inch (0.91 m) sewer tile, open at the bottom, makes an excellent lining.
This is cheap insurance against the possibility of needing an expensive and damaging ditch-blasting operation, or an elaborate waterproofing job.
Waterproofing. If there is no downhill point or stormwater pipe to which subsurface water can be drained, and location and soil type indicate the probability of groundwater, the basement walls should be waterproofed. This can be done more thoroughly and economically during construction than afterward.
Condensation. Before undertaking any considerable expense to stop leaks into a basement, it should be found out definitely whether they exist. Condensation may make substantial amounts of water appear on the walls and floor. If the trouble occurs in hot weather, it is probably condensation; if in the wet season or during heavy storms, it is most likely leakage. If a piece of cardboard is secured against a suspected spot, it will get wet on the wall side if it is leakage, and on the room side if it is condensation.
Condensation may be checked by coating the wall with cement plaster or some other coarse, absorbent material, or by running an electric dehumidifier in the room.
